If they're wired up in parallel they'll both keep the same charge level, basically becomes one larger battery. I believe the Tender is capable of doing much larger capacity batteries, so theoretically it should work.
The issue I see is if the batteries aren't in the same condition. If one has a problem it could end up ruining the other.
You can get a Tender JR for under $30. You'd spend a good chunk of that rigging up something to share a tender. For that matter, you can get an extra vehicle side connector - just move your tender every couple weeks.