1993 Pontiac transport van has two heater cores are they interchangeable?

No, 1993 Pontiac Transport vans did not have two heater cores that were interchangeable. While the van might have multiple heater units (for example, one for the driver and passenger area, and potentially another for the rear passenger area if it was a longer wheelbase model with rear seating and climate controls), they were almost certainly different units. Their design, size, and plumbing would vary depending on their location and the specific climate control system in the vehicle. You'd need to find the correct part number for the specific heater core needing replacement.