I can't think of a case where seamlessly having two different targets like this has worked - particularly if there's pressure to really support all of the features of both.
Yes it does happen that some features get neglected, but we're talking shell scripting here. They could hit the 80% use case and it would be totally useful.
[1] http://haxe.org/
For those kinds of simple tasks, the overhead of fully understanding the nuances of bash _and_ bash is way more trouble than the inconvenience of having fewer features available. This is not for bat/sh lovers, it's for those who have to use bat/sh even if they'd rather not.
Having said that, I wouldn't use batsh myself until it does something sane with errors (at least the equivalent of `set -e` in bash).