The popular 2-tier stand is the Ultimate Support Apex, which holds 2 (or
three, if you buy an extra tier)
keyboards, and best of all, folds down into 1 piece for portability.
There's a studio version called the Deltex, it's cheaper but doesn't fold
down.
If you're into da old skool, you might be into Invisible -brand stands.
Very sturdy. I don't know if they make them anymore.
Don't get those "X" stands with the extra deck. "X" stands (not a brand,
many manufacturers make them, they just resemble an "X" when set up) are
good for one keyboard, but put two and they come out to be rather flimsy.
If you're just in the studio, you can also get those (real old skool!)
Ultimate Support A-frame stands, which can hold 3-4 keyboards. You can also
use them for live situations, but they take a while to set up.
Elson