I've seen some fish tank dividers that might work ok for that.
Otherwise you might be able to cut your own piece of plexiglass or use cut ceramic tile. Maybe drill a couple of holes in the top to hang it securely from the top and keep it from falling over. Also anchor the bottom with rocks.
The advantage of ceramic tile is that separated geckos wouldn't get too upset seeing each other if that's an issue.
The divider may not have to go all the way to the top of the tank. Crickets always figure out how to get around them anyway. Just go high enough to ensure you'll never have a gecko getting from one side to another, then add a few more inches for good measure.
A wood divider might work too though it may be harder to keep clean if one decides to use that corner as their defecatorium.
Anyway, that's what I'd do.