going to reed flute cave and from Guilin to Yangshuo in a single day

I hope you are referring to the bamboo raft instead of the sightseeing cruise, because the cruises are only in service around 09:30 - 10:00 each morning. Then your plan shall be feasible:
after visiting the Reed Flute, take a taxi to Yangdi. There you can rent a bamboo raft to Xingping. Upon dropping off, walk to Xingping Town and transfer to a bus to Yangshuo.