GST ON GTA service RCM OR FCM

an organisation has 2 branches having same PAN number, one in assam & another in west bengal and hold state wise gst registration number for both places.

in assam, he already opted Fcm on GTA and charge GST @ 12%.

in West Bengal, he want to opt RCM on GTA @ 5% GST. Can he do so?

pls help with supporting section/rules of GST.

THANKS