How to fit a 2 way valve to a 1.5 inch pipe

A few more details would help. What material is the pipe made of and what function, supply , vent, drain. Not sure what you mean exactly by a two way valve, can you explain it further.

Hmm 2 way valve-gate valve? Ball Valve? Threaded pipe? Plastic Pipe? If it's a threaded valve and a pvc pipe, use a 2" male IP to 2" pvc and use a PVC busing to 1.5" Otherwise, you'll need to be clearer as to what you wish to accomplish. Post a picture?