Aggh! Why?!
Even assuming the leo is very mellow and has no bad intentions toward the banded, think about this:
1) You don't know how the banded will react to the leo. He might be aggressive, he might ignore him, or he might be scared s***less; picture yourself living in a small enclosed area with something 4 times your size and 8 times your mass;
2) It's feeding time, and you've just put 6 large and 6 small crickets in the cage; suppose the leo lunges for a cricket and the banded happens to be in the way; maybe he get's stomped, maybe he gets bit;
3) There's one large cricket that managed to avoid getting eaten and has hidden in the cage. Later on he starts getting hungry and comes across your little banded sleeping. How big a bite can a cricket take? (Okay, I admit this one is a little far-fetched, but I couldn't resist 
)
In my opinion, it's a bad idea to house together different species of such disparate sizes, both occupying the same biological niche and essentially directly competing with one another, unless you've got something way larger than a 20 gallon (think 120 gallons and up).
>>A very mellow Leopard gecko with a Banded gecko once they're adults? In 20 gallons
>>
>>The Leo was a sickling rescue so he's healthy now but real mellow, missing most toes/nails so he can't climb at all, and just chills in his tank but is active when I take him out (he wanders ALL over the living room but never bolts!).
>>
>>The leo's been in with other baby leos and just hangs out in a pile with them.
>>
>>I'm guessing no...but perhaps there's a chance?
>>-----
>>Too many Leos
>>1.0 feline "Spot"
>>0.1 canine "Tika"
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Chris
2.0.0 Coleonyx variegatus
1.0.0 Gekko vittatus
0.0.1 Tarentola mauritanica
0.0.2 Rhacodactylus ciliatus
