The Resistance welding controller goes beyond just maintaining the current.
1. Its job is to control all resistance welding parameters that include current, Pressure, heat, Water circulation for cooiling, 2. Provide various cycle timings such as Squeez, Precompress, Forge, Weld cycles, Hold, Index, Index delay, etc.
3. Control Outputs such as Solenoid valves of various cylinders, Index Output, Motor Output for seam welding machines.
4. Take inputs from Foot Switch, Thermal switch, pressure switch, seam cut off etc.
5. Provide a good user interface for programming, executing weld schedules, Provide diagnostic routines for trouble

Since you are very particular about the precision of the welding, I would definitely farm the remaining processes out to other processors. You are, in effect, multitasking and using one processor with a "multitasking OS" would ruin your precision.

I am currently finishing development of a unit that has 4 processors: a LPC and 2 SILabs f12x connected by IIC and one Cypress USB master processor connected to one of the f12x by SPI.

Every time critical process is taken care of IMMEDIATELY since only one exist for each processor. For such multiprocessing in combination with critical timing it is essential that you use hardware interfaces only, thus the SPI connection for the Cypress that does not have hardware IIC. The f12x that sit as a master does not have anything fime critical and can thus enter waits for the other processors to finish the time critical operations with no deterimental effect.

Erik