Seting up a Cube tank, need advice

[prosnow]

i'm currently getting a cube tank built, 20x20x20
and am going to be using a 10 gallon tank as my sump,
i was wonder what kind of pump i should use as a return i was thinking of something along the lines of a mag 950, and split that 2 way so i can get some extra flow out of it, or do u think this is a bad idea? i am hoping to not put any kind of powerhead or anything else in the tank itself,

any suggestions are welcome,

thanks Matt