  The various applications using Distributed Ledger Technologies (DLT) or
blockchains, have led to the introduction of a new `marketplace' where multiple
types of digital assets may be exchanged. As each blockchain is designed to
support specific types of assets and transactions, and no blockchain will
prevail, the need to perform interblockchain transactions is already pressing.
  In this work we examine the fundamental problem of interoperable and
interconnected blockchains. In particular, we begin by introducing the
Multi-Distributed Ledger Objects (MDLO), which is the result of aggregating
multiple Distributed Ledger Objects -- DLO (a DLO is a formalization of the
blockchain) and that supports append and get operations of records (e.g.,
transactions) in them from multiple clients concurrently. Next, we define the
AtomicAppends problem, which emerges when the exchange of digital assets
between multiple clients may involve appending records in more than one DLO.
Specifically, AtomicAppend requires that either all records will be appended on
the involved DLOs or none. We examine the solvability of this problem assuming
rational and risk-averse clients that may fail by crashing, and under different
client utility and append models, timing models, and client failure scenarios.
We show that for some cases the existence of an intermediary is necessary for
the problem solution. We propose the implementation of such intermediary over a
specialized blockchain, we term Smart DLO (SDLO), and we show how this can be
used to solve the AtomicAppends problem even in an asynchronous, client
competitive environment, where all the clients may crash.
