Never tried doing this but I figure that what you could do is sew a mitten hook onto the bottom corners of the poncho. What you could do then is run your tarp's ridgeline through your ponchos hood and cinch up the hood. Then attach the mitten hooks to the webbing at the bottom corner of your tarp and use some cord to stake out the sleeve. This would form a sort of "beak" for your hammock, much like a ten's vestibule