I have no strong opinion about which one should be the "leading" tag here, but I think it'd make sense to merge them.
I don't think it makes sense to have a tag for the shell builtin
source specifically. It's not that special a feature that it wouldn't just fit under shell-script (or bash, which is what most of those are tagged with anyway). Besides,
source isn't even a standard command. For POSIX-compatibility, it should be dot-command instead. Though source-command already exists, it's only been used by Jeff.