Dual electric motor drives are done all the time in my area, where larger hp's are required on single phase power, so dual 7.5's or dual 10hp can be started individually and get more hp than one 10hp single can. Grain bin draw outs are all dual drives and many load out augers as are some direct drive dual fan bin fans.
Not sure why you'd want dual 5's though, you can start one ten on single phase power unless likes already been stated, you had the motors.
Curious where you got a vfd that cheap?