Tigg
Those Petco cages are fine for what they are, but not sturdy enough to stack that way. I'm not even sure it's possible. As I recall, the bottom is not removable without collapsing the cage. Plus I'm pretty sure you're left with a gap where the tray used to slide in (for the cage on top, as you would naturally also have to remove the tray).
A better option would be to just get a larger flight to start with. I have the
Prevue F040 flight cage, currently on sale for $100 on Amazon.
I see the Petco cages are on sale right now for $37.50 each - while the Prevue cages will cost you $25 more, Its just far more convenient and way way WAY sturdier. It's a better cage, well worth the additional $25 expense. And, should you ever really get the bug, you can buy another one later and attach it to the first and have a 5' flight cage, LOL! (That's what I did with mine)
As far as spraying water and seed hulls - I hang a sheet over the back of the cage to protect the wall. I have never found poop outside their cage, not even the parakeets do that - but they do shed feathers and seed hulls, and when I give the finches their bath, they do spray that around. The sheet catches the vast majority of anything like that, then I just take the sheet off and wash it once in awhile. The seed hulls do not go to far afield, I just vacuum that up with a handheld vacuum once in awhile. I have a Prevue F070/F075 (whichever one does NOT have the stand) that sits in a large garden tray which catches 95% of any scattered seed.
I got the garden tray from GreenhouseMegastore for $15. If you stick with a 30x18x18 cage that might be a useful item for you.
For some reason the seed scatter is less in the dual F040 setup than it is in the F070/F075. That may just be a difference in the birds - I've got my parakeets in the F040s and they don't actually get a lot of seed by itself anymore since I got them switched to a lotsa-greens and bird bread diet. They are now 80% fresh veggies and 20% seed instead of their previous 100% seed diet, so there is less seed from which to scatter hulls - plus about half what I give them is hulled and sprouted seed from Trader Joe's, so ... even fewer chances to scatter hulls outside the cage. The feathers though - they go EVERYWHERE, LOL!