  We study many-body quantum coherence and interaction blockade in two
Josephson-linked Bose-Einstein condensates. We introduce universal operators
for characterizing many-body coherence without limitations on the system
symmetry and total particle number $N$. We reproduce the results for both
coherence fluctuations and number squeezing in {\em symmetric} systems of large
$N$, and reveal several peculiar phenomena that may occur in {\em asymmetric}
systems and systems of small $N$. For asymmetric systems, we show that, due to
an interplay between asymmetry and inter-particle interaction, the coherence
fluctuations are suppressed dramatically when $|E_{C}/E_{J}|\ll 1$, and both
{\it resonant tunneling} and {\it interaction blockade} take place for large
values of $|E_{C}/E_{J}|$, where $E_C$ and $E_J$ are the interaction and
tunneling energies, respectively. We emphasize that the resonant tunneling and
interaction blockade may allow creating single-atom devices with promising
technology applications. We demonstrate that for the systems at finite
temperatures the formation of self-trapped states causes an anomalous behavior.
