Went looking on You Tube to see if there is anything showing a poncho tarp and hammock:
The comments are interesting.
At about 11:35 he mentions that the ends of the hammock are outside the poncho tarp.
Based on this, I'd say that a poncho is less than optimum for use with a hammock for rain protection. It's probably a sensible option for ultralight ground dwelling in conjunction with a bivy sack.