Yes, the seat base and mounting points are the same, so a T6.1 heated double bench will physically fit a T5.1.
The thing to check is the wiring. VW changed parts of the electrical architecture on the T6.1, but the heated seat connectors themselves appear to be the same. Several members who've swapped between T6 and T6.1 heated seats report that the heating works plug-and-play with matching OEM heated seats and no coding required.
Before buying, I'd compare:
- the under-seat connector(s)
- seat belt buckle/occupancy sensor connectors (if fitted)
- whether the donor bench came from a van with factory heated seats.
If the connectors match, I'd expect it to work. If they don't, you may need to swap part of the loom or adapt the wiring.