Where to start?

[Jonathan]

Would VFD or PWM help at all?

The spindles controlled by a VFD are induction motors and the VFD aims to get constant torque down over the full speed range. It doesn't quite manage that, particularly at low speeds. The problem is Power=torque*angular velocity, so as you reduce the speed, with torque constant, the overall power available reduces linearly, hence at 1/10th the rated speed you'll have hardly any power left. The same sort of thing is true for other types of motor since fundamentally the torque is proportional to the current in the stator and you can't increase that above the rated current.

One option is to just get a much higher power spindle than you need for just PCBs, then it could have enough power at low speeds for cutting steel. That may work out more expensive than two spindles though...
Another option is to use tools which tolerate being run at a higher rpm than one would normally use in steel, such as these:
X-5070 Blue Nanograin Coated Solid Carbide (Hardened Steel HRc 50-70) - Cutwel Products

If you set up flood coolant then that will help use a higher speed which might be enough to get into the range for normal carbide tooling.

You could make two spindles with RC motors as they're relatively cheap and can be made for very high speeds which is ideal for cutting PCBs.