For one cross it would require two wooden beams (nailed or tied together with a rope). And there is a bigger possibility it collapse, because of the junction.
Why wouldn't a single wooden beam be enough?
Here is an earlier discussion of the shape of the crucifixion device that covers a lot of this. Although I'm sure there is more that can be said.
TL;DR - A simple stake often was good enough. "Cross" is an English word that came much, much later, so we should be careful about projecting English meanings back too far. The Latin "crux" did not have the same implications, and the Greek stauros is more stake-like in its connotations. There were a lot of shapes that could serve the purpose.