"Jay" wrote in message
Apparently there was some rule that said you could only have a
single engine?

Not that I know of. The point was that I was trying to identify the
engines used by these aircraft, and the Italians had assigned the
single identifier "AS6" to the two engines, whose crankshafts rotated
in opposite directions.

In WW2, the British firm Napier developed the 24-cylinder Sabre
engine (effectively two in-line 12-cylinder engines!) for the
Hawker
Tempest. On the bench, this engine easily developed 3,750hp.

And an in-line engine arrangement does wonders as well for drag!

Especially liquid-cooled in-line engine(s), which have little frontal
area.

Someone tell me those aren't surface radiators on the Italian
Machii-Castold MC.72 seaplane?!?!?!?

They are indeed radiators. In fact, the MC.72 was at the time called
a "flying radiator" because of the amount of cooling needed by those
two engines. ;-)