is there a way to set it so that Teleporter A only teleports if Teleporter B is empty?
Been trying to figure this out with circuits but to no avail.

Nvm, figured it out
For anyone else that's curious the way i did it:
-Set Teleporter A with Link ID X (or w/e), Set Teleporter B with Link ID X
-Have inserters on Teleporter B empty all items into a chest
-Connect chest to Decider combinator (DC). output of DC to Teleporter B
-Decider combinator conditions are that when chest is empty, send the planet signal of Teleporter A to Teleporter B
-Teleporter B turns on (since now it's empty) and transfers contents with Teleporter A (which is now full)

That's a clever solution. I think this is a good suggestion regardless, though, so I've added the ability to check the target's state and only teleport if it is empty (or full), the default behavior is "Any" which ignores this check and functions like it does now.