I'd like to chime in on this with the 'old guy' perspective.
Warning, this may not make sense since it comes from the 'other' side of the fence.
Some of us who grew-up in the '50s-'70s would watch with fascination during auto tire changes, seeing the tire guy do the 'balance' job with the wheel setting on a bubble-level balancing rig, carefully placing wheel weights about the rim to center that indicator bubble. Or later, on those new-fangled spin-balance machines. And then, if you had the opportunity, watch (or perform, if you were in the business) the process of crankshaft balancing.
All this 'balancing act' obsession would give one a love of weight-distribution symmetry, equal sized spokes on wheels, equal sized blades on propellers, equal sized shoes on your feet. This kinda lockstep thinkin' can blind you to alternatives.
Then, along comes this lopsided/wobbergobbled doo-dad called a "rephased engine", with its twin cylinder crankshaft twisted out of idealistic symmetry into a 90° wobbly-gonk.
???
It sure knocked ME for a loop. Sure seems that it would vibrate itself into pieces.
Then, I remembered my ancient '20s-'30s engineering handbooks, with their exhaustive text/formulas/tests on the design of flywheels for use in industrial machinery.
Long story short, the purpose of the flywheel is for the storage of kinetic energy, to be dispensed as necessary to perform some function. The design criteria involves calculating energy containment, energy release, and recovery. The flywheel naturally slows during energy release, and the designer must account for this change in angular velocity, keeping enough in reserve, and ensuring that the bearings and support structure can handle these changes.
Enter the single cylinder engine. The flywheel is the energy storage, and its angular velocity actually changes (faster and slower) as it exchanges kinetic energy with the reciprocating parts (piston, wristpin, rod). When the piston is at TDC or BDC, its kinetic energy is zero, and the flywheel (crankshaft) is at its greatest rotational speed. Conversly, when the piston is near mid-stroke, its speed and kinetic energy is maxed, and the flywheel (crank) is at its slowest.
This varying angular velocity of the crankshaft is significant, and must be accounted for in the main bearings, since they act as the fulcrum point between the mass-center of the crank and the mass-center of the reciprocating parts. This is the true source of rectilinear motion vibration.
Add another crank throw (second cylinder) in the same phase (orientation) and the effect doubles.
The rephase configuration forces these two flywheels (crank halves) to attempt to cancel each other's varying angular velocities, by distributing the varying kinetic energies between them, significantly reducing the spin speed differences, and reducing loads on the crank main bearings, hence reducing vibration. It's like the natural smoothness in 90° throw straight-8's, not so in flat cranks. Of course, this new energy exchange must be transferred through the crankshaft center pin as severe twist/torque moments.
In the idealistic/frictionless analysis, this rephasing has no effect. But, add some reality, and this rephasing will demonstrate some degree of increased output, both by reduced main bearing loads and by reducing irreversible energy losses from vibrating your bike (and your gelationous you).
Hook-up one of those old '20s weight-reducing belt vibrators to your bike (or a commercial paint shaker), adjust its output to duplicate the vibrations you currently receive while bike riding, and that would represent the amount of energy that is robbed from your engine...