In most cases a single motor is better than one for putting around.
I have a pair of 2.2hp Mitsubishi engines on Staton friction drives on "The Dragon Lady". Front roller is 1.25", rear spindle is 1.5". These engines are independent of each other. They do not need to be synchronized or run together. However there is less resistance drag and obviously more power if both motors are running.
(On "The Iron Dragon" it has the same engines w/front friction drive and Staton rear chain drive w/18.75:1 gearing. However, it has less undesirable resistance issues because of the chain drive.)
There is a very significant amount of resistance drag when both engines are disengaged om "The Dragon Lady", especially when walking or pedalling the bike. At slow speed you might have to stand up to pedal on level ground.
Of course this is easily remedied by lifting the roller(s) off the tires. Then the bikes would pedal easily, even with the extra weight of two engine assemblies.
The rear engine suffers the most from resistance drag when the front engine is idling. To overcome that I devised a lever which would lift the friction roller off the front tire when the engine's clutch was disengaged. It worked well but I rarely used it because both engines were engaged and disengaged so often.
When I removed the front engine of "The Dragon Lady" to replace the friction roller, I noticed that the bike's low speed handling was much better. With front engine you have that extra weight hanging offbalance to left side. That's a problem inherent to front engines, especially if the engine is not centered on its mount.
There was less friction with front engine missing. The rear engine could work better with its 1.5" roller. There was less weight on the bike and the low-speed/walking speed handling was significantly improved.
Once the bike gets under way, the bike's unwieldy front end is not a handling issue. It is like having a supercharger providing MORE than 100% extra hp at any speed. I'm saying that more than 100% power is available because one engine's friction roller is smaller. That engine develops more hp/torque at any speed because it's spinning at a higher rpm than the engine with the larger roller.
If traveling at 25 mph using the rear engine, when you throttle the front engine you're adding an extra 2.2hp and neither engine is being pushed beyond its normal limitations.
Using two engines with identical oil/fuel ratios has less problems when installing an auxiliary fuel tank. I mounted a Happy Time tank on the crossbar of the men's bike("The Iron Dragon") and the downslope bar of "The Dragon Lady". Both engine tanks are replenished by the aux tank and fuel capacity increased by 100% per engine.
Contrary to popular belief, both engines don't have to be synchronized or used together. Each has their own throttle. On "The Iron Dragon" I start off using the rear chain drive to minimize front tire wear from friction roller. Once rolling, any combination of engines can be used. (Of course the one I like best is both engines running at all times.)
I am in the process of changing front friction roller from 1.25" to 1.375" size. Front engine is screaming at top end; the larger roller should save the engine and allow higher top speed.
Because two engines are being used, no significant loss of power should be noticed at low/midrange.
When caught in a continuous downpour six miles from home, I disengaged the friction engine and used chain drive to motor home safely.
Also, when one engine konks out, the other one brings the wounded bike home at 20-25mph.
If each engine was running on the back wheels of a trike, then they would need synchronizing.