Now that's a real thought provoking question!

My initial and un-scientific guess it that you would get something less than
the paper calculations might show because you have the added physical drag
from the second motor that MIGHT be greater than that from a single motor.
I would suspect that the actual displacement of two motors would be greater
than that of a single motor I.E.; more stuff in the water equals more drag.
Secondly, if the single motor is rated at a higher efficiency than the two
motors are, you've lost ground in battery life. Conversely if the smaller
motors are more efficient, you're ahead of the hounds.

As I said... What a thought provoking question.

Here's bettin' that this stirs up a lot of opinions and will be fun to
follow!

Don Dando