OK, let me get this straight. You are moving water from the 90 (main tank) to the 29 (fuge) to the sump, then back to the tank.
Gravity fed overflow from the 90 to the fuge, the fuge will require a hang-on overflow and it will have to be higher than the sump. For gravity to work properly, the 29 will have to be raised off the ground and IMO
, the bottom of the 29 should be higher than the top of the sump. The water from the overflow on the 29 will be fed to the sump, which should pump the water from the sump to the 90. This is the only pump that should be working, and all overflows need to be rated to keep up with the output of the pump. Make sure there are no bubbles in the U/J tube of the overflows and you shouldn't have a problem.