You can't complete your multi-engine rating in a centreline thrust aircraft, as you have to demonstrate asymmetric thrust recovery and control.
A C336 or C337, however is definitely a multi-engine aircraft, and should be logged as such. It has TWO engines - count 'em - TWO.
("Conventionally" configured and centreline thrust multi aircraft both fly around with symmetric thrust in their operations, there is no difference in normal operations.)