production engines with multiple crankshafts, while rare, do exist. Examples would be the Ariel Square Four and the Rotax 256 tandem 2 cylinder two stroke. Other examples are the opposed piston designs where there are two cranks with the pistons meeting head to head to form a combustion chamber. So . . multiple cranks would not be a hard and fast rule for multi engine definition. However those engines started out as one engine from their manufacturer.
. . . in the case (bad pun) of the "V8 Busa" the cylinders and heads are from two engines but there is only one crank. That one might be in the gray area as to being one or two engines.
However the special two litre engine(s) being built from two bike engines are IMHO "two engines."
Hummm . . . . . If you aren't allowed to run two engines in anything but streamliners (car and bike), Sidecar streamliners, and open (A) bikes. . . . I guess that means -by definition- that all other classes
must run only one engine.
If you are fool enough to still be reading, . . stick with me here . . .So . . if you must run one engine . . . then . . . . does that mean that you cannot run less than one engine where mathematically "one half" is less than "one" . . . .meaning that George Fields' half hemi in the comp coupe is not legal! Otherwise one half would be equal to one. (fuzzy math).
OK . . . I'll go away now . . .
P.S. Just kidding George