the reason this is not recommended is that if the going up powerhead stops working, your fuge will flood and spill water all over the floor until the tank water line drops below the level of the powerhead in the tank. also you could get a spihon going in the case of a power outage, also causing a flood.
if the going down powerhead stops working, the one pumping up will pump the fuge dry, burning its self out and causing everything in the fuge to die from not being in water.
also, 2 powerheads of the same size will not move the same ammount of water, as there will be head pressure on the one pumping up, but not on the one pumping down.
your only two real options for getting water safely out of a tank and into a fuge is either by using an overflow from the tank down to the fuge, or have the fuge above the tank, and have it drain via gravity through a bulkhead hole back into the tank. in either of these circumstances, if the pump goes out, you will not have a flooding problem.
believe me i tried for a long time to figure out a safe and cheap way to do this using powerheads, and its just not feasible.
HTH
p.s. if you can not drill your 10 gallon tank, you can always go to walmart or office depot or what not and get a clear plastic file box. the one i bought when playing around with fuge design was about 8 bucks, was see-through, and held about 8 gallons. this you can cut a bulkhead hole in with a drill, i used a dremel even, meaning ou dont have to deal with drilling glass. bulkheads are 5-10 dollars from a plumbing supply store, online, or a larger home improvement store. powerhead about 20 bucks. an elbow and tubing from the bulkhead is like 4 bucks... so you are looking at about 30 bucks for a fuge, not including light... thats pretty cheap and its an easy diy.
lotsa help to be found on fuges in the equipment and diy section if ya got more questions.