Relational tables with same content, custom solution
I would keep them separate. Although the details differ, your circumstance smells a lot like the One True Lookup Table antipattern. By combining you will have problems defining foreign keys (which parent to point at?), amongst other things. Indexing strategy will essentially be about spliting the combined table into its constituent parts. And so on...

The justification for combining would be if Foo and Bar are sub-types of some more abstract entity type and all comments could hang off this super-type.