Tom,
You are confusing the terminology. VFDs are often derated when a three-phase VFD is fed from a single-phase source. A three-phase VFD has six rectifier diodes on the input. When used with a single-phase source, only four diodes are used, limiting the overall current. As an example, a three-phase 3 hp VFD may be derated tor 2 hp when supplied from a single-phase circuit.
No one is addressing motor derating because it is not something one does. One can apply a 3 hp motor at a capacity below its rating (2 hp for example), but it is still a 3 hp motor. Likewise, an unloaded 100 hp motor may be "consuming" 5 hp in losses just sitting there under power, but it is still a 100 hp motor.
On the other hand, manufacturers specify motors with a Service Factor (SF). The average motor may have a service factor of 1.0, meaning it can operate continuously at 100% of its nameplate rating. Industrial and other special motors may have a service factor of 1.1, 1.15, 1.5, which correlate to continuous operation at 110%, 115%, and 150% of nameplate rating. A 10 hp motor with a 1.5 SF is essentially a 15 hp motor in disguise.
In your case, if you want to run both motors at 2 hp, fix the VFD settings for the 2 hp motor and a standard volts/hertz (non-vector control) mode. If you want to keep both motors at rated hp, the easiest way is to use a mechanically-interlocked reversing motor starter with an auxiliary contact. The "forward" contactor can power the 2 hp motor, the "reverse" contactor can power the 3 hp motor, and the auxiliary contact can be used to signal the VFD to switch motor parameters. One selector switch can control everything, although it would be smart to use some relay logic to keep either contactor energized until the VFD has decelerated.