GST and job work

Kolkata based company places 2 separate POs on 2 different parties in Chennai. Party A will supply Mandrel . But to avoid transportation, can we ask them to send to Party B , who will then install Mandreal on Gear Box ( purchased under separate PO) and then send to Kolkata.

About GST credit and payment to B  , there is no issue. But how to handle transaction and GST with Party A

Kindly guide.