soft start

Larger idler motor on the phase convertor would probably do it, as would starting the vertical spindle before trying to start the horizontal so the vertical motor acts as an additional idler. The VFD is certainly the best way to go, and you could with appropriate safety interlocks multitask a single VFD to run either motor (one at a time).

Karl, I once owned a hydraulic shift Cincinati knee mill with dual spindle motors as well. Lovely machine. Depending on the motors and the number of wires available, you could use a two way start switch where you start the motors in a "Y" configuration and then, when up to speed, switch them to delta for running under load, but you need 12 wires not 6.. Steve

There are two easy and obvious answers:

2) Use a VFD (whether it is easy depends on the control system).

How about an idler pulley/simple style "clutch"?

Since starting is marginal, extra capacitors across the motor leads may help. You could try just one capacitor across the same two leads as the main capacitor in the RPC and/or equal capacitors across all leads to help the start power factor a bit. The capacitors should not have to be disconnected after starting.

Don Young