I have seen a video of a byzantine rite of priestly ordination and the bishop only laid one hand on the ordinand. So im assuming that a bishop doesn't need to lay both hands on the ordinand in order for the ordination to be valid.
Yes, the Eastern Rites always used one hand, and the Roman Rite did for a few centuries also. When this controversy came up, I went to The Catholic University of America and located copies of the
Pontificale Romanum over the centuries and found that for several centuries the Roman Rite stipulated "dextram" (the right hand) in the rubrics.