I think it would be more of an eyesore in your tank to increase the plumbing size to connect 2 FX 5's together just to eliminate 2 inputs and 2 outputs in your tank. Plus, it is not recommended to connect two canister filters in series and have them both pumping out water.
You could always just use the second FX5 for bio media only. Use appropriately placed tank decor to hide the intakes if you don't like the way they look in the tank.
You could always just use the second FX5 for bio media only. Use appropriately placed tank decor to hide the intakes if you don't like the way they look in the tank.