Q: Starting a sentence with two subordinating conjunctions Is it grammatical to start a sentence with two subordinating conjunctions? For example:

Because if it rains tomorrow, I will get wet, I hoped for a sunny day.

It seems wrong to start a sentence with "Because if..." and the last two clauses feel like a comma splice, but I can't figure out what's actually incorrect about it.

A: This sentence is fine - though as it stands, I think the punctuation isn't doing it any favours. The main clause here is I hoped for a sunny day. This is being modified by an adjunct headed by Because. The superordinate phrase in the adjunct is Because I will get wet, which is itself being modified by another adjunct headed by if, namely if it rains tomorrow. All of this is perfectly permissible.
However, the punctuation here is misleading, because in natural speech, we would tend to parse the sentence so that there were two main identifiable sections; the large clause headed by Because which contains the conditional adjunct: 


*

*Because if it rains tomorrow I'll get wet,


and then the superordinate clause that this modifies:


*

*I hoped for a sunny.


There will probably be two intonational phrases ('musical segments') in the because-clause, but these will run together with no discernible pauses. There will be a very discernible pause after wet, and then the second major section of the sentence will probably have one more intonational phrase marking out the main clause. This will probably end on a high fall nuclear tone, helping indicate that the utterance has terminated. 
In the example, there is a comma between tomorrow and I will. This will tend to make a reader feel that there is a pause here, and it will slightly misdirect them. What they are waiting for is the pause caused by reaching the end of the clause headed by Because, not any pause from the internal clause headed by if. Because of this, it would be better to remove the comma after wet. 
However, the story does not end here. The intonational tunes in the larger because-clause are quite sophisticated. Here's how. There will be one tune for the segment Because I'll get wet. This tune will be interrupted without any pause after the word Because. The pitch will then suddenly drop and a completely new tune will run for the duration of if it rains tomorrow in this lower key. When this finishes, the first tune will instantly resume as if nothing at all had happened and run until the end of the word will. 
The point at which the first tune is interrupted seems to deserve a comma in written English (in the way that this is marked out in non-defining relative clauses). This first comma would not misdirect the reader, because they are not reading it as the end of the because-clause. So although the sentence would be easier to read without a comma after will, it seems to require one after Because. 
In spoken English the sentence will be vastly improved with a contraction of the will not. If not in a formal style, then this would also be well reflected in the writing.
[Lastly, to my Britannic ears, the sentence would be slightly improved with a past perfect form instead of hoped, but this may not be the case for all readers. Hence the brackets here.]
The reasons for using this rather ornate phrasing is undoubtedly one of information packaging. The most important factors are what has already been talked about or alluded to, and what the speaker wishes to emphasise. Both new information and information that the speaker wishes to emphasise will tend to be placed at the end of the sentence, where it will attract more focus from the listener.
In short then, this perfectly fine sentence might be considered more leniently (imo) if written thus:

Because, if it rains I'll get wet, I'd hoped for a sunny day.


A: 
Because if it rains tomorrow, I will get wet, I hoped for a sunny day.

Okay. It's fine-ish.
The tense seems a bit odd. You are talking about a possibility for the future (relative to now?) and of a hope in the past.
That's not wrong. It would make sense for example if someone had just told you (today) that there was a forecast of a strong chance of rain. The rain remains just a future possibility, but the hoping was done in the past, so all the tenses work logically.
It's just not a very likely combination of tenses, so standing on its own, without the context of such a forecast, it strikes me as odd, where it wouldn't just after such a forecast or some other reason for the unusual tense combination.
So let's change the tenses just so I'm no longer distracted by that!

Because if it rains tomorrow, I will get wet, I hope for a sunny day.

Can I follow that? Yes.
Is my understanding of if what you wanted it to be? I'm pretty confident it is.
Do I at any point get tripped up, and have to mentally backtrack to understand you? No.
Can I see any grammatical problems? The first comma isn't wrong, but isn't necessary either, and I think you'd be better of without it, but I don't see any problems.
Can you start a sentence with two subordinating conjunctions? I'd say "yes" as a general point of theory, and I'd accept the above as evidence to back it up.
Is it a good sentence? Ah…
It takes me a while to get to your point. 
Now, that's not in a terrible way. I don't get half-way through your sentence, realise I misinterpreted a clause and have to go back and start again.
But it's generally not a good thing either. There's a lot of build-up to just the hope of a sunny day.
But it's not always a bad thing.
It happens with any Because sentence, and the if adds to the effect. Consider:

Because the Origami didn't work
We gave you dancing lessons
Because the dancing lessons made you bored
We bought you a guitar
Because you said
It made your fingers sore
We found you a pony
Because the pony broke your leg and ran away
We got you drums

Here the fact that putting the subordinate clause first leads us along for a bit before we get to the main clause is the whole reason this song works.
Your "Because if" structure increases the "leading along" quality, but the pay-off doesn't seem to be worth it.
And of course example sentences are rarely those with the best punch. (It's hard enough to think of a good sentence when writing, or a valid example when examining grammar, so doing both together is really tough). The actual form of "Because if…" I'd say is fine, I'd just better get something after it's done leading me through the subordinate clause.

A: The sentence makes sense in response to a question, such as "Why are you so eager to open your weather app?" or "Why are you looking for the weather report in the paper?" Its construction relies on an overt or implied question, though, so I wouldn't open with it.
