If you use 2 pumps, you have to make sure there's no pressure in the container.
You can do that by putting the container higher than the water level. Otherwise,
the inflow will always be greater than the outflow, creating pressure, and water
will come out of the container. You need to experiment on where to put your filter.
If you can secure the lid of the container very well, you can deal with the pressure
problem. But I found out that water pressure is difficult to deal with...
Here's the canister filter I built, using 1 pump:
http://www.redearslider.com/forum/viewtopic.php?t=21557
Notice where I positioned the canister. If the pump doesn't run, water will come
out of the lid of the canister because of the water pressure. If I raise the
canister, the total flow will lessen.