You'd have to block the drains in the 90g and use a HOB
overflow on the 120. Plugging the drains can be done a couple ways.
1. Silicone a piece of glass/acrylic/plexiglas over the top of the overflow. Maybe a little crude, but effective.
2. Use plumbing parts from the hardware store. Easy enough.
You may be better off using the 120g as the sump after all. I think I'd be afraid of the drains still leaking. If the tank is drilled for returns too, you'd definitely be best off using the 120g as the sump.