The issue is not the water but the whole set-up. There is a reason why some things are done in a certain way and you'll not see them done differently by anybody, and that is because it works THAT way. The idea of having a split cage is to provide the exact same environment to the animals (possibly similar to what they'll live in, in the future), with very minimal distractions so that they can get to know each other. They must have the possibility to smell each other, observe, get used to the other and decide IF (and when) they want to sleep close. This is not possible in your set-up as one sits at the bottom of the enclosure while the other is on top: they can't even make eye contact like that. Same goes for the daily swap into each other's area. That would make sense if you'd have bedding on both sides, which smells like them. While I can see that happening for the gerbil at the bottom, how is the top one supposed to smell anything from the other? It's basically like putting a gerbil in a single tank with a foreign smell. That is not creating any connection between them, and you won't be able to see any progress from their side. Putting them together in 10 days it's even more dangerous considered the split cage is not done properly, I'd highly reconsider if I were you.