Oh, goodness, I really don't think that's how it should work.
I think that seed selection in the seed rooms should only be set once you leave the room.
1. Enter a seed room
2. Pick a seed
3. Enter test room
4. Try out seed
5. Leave test room
6. Grab other seed, first seed grabbed returns to its pedestal.
7. Repeat until you leave. When you leave, that's the seed you picked, the other seed disappears if you return to the room.
That makes it all way more discoverable without having to explain how it works. If you enter the room and just try and grab both seeds, it's self evident you can't (because one seed returns when you grab the 2nd) and the "pick and seed and then test it out" is super intuitive.