Getting like charges to attract and opposite charges to repel

There’s a simulation I want to do that requires particle system A to attract particles in A and repel particles in B, and of course B attracts particles in B and repels particles in A.
I thought that I could do this by using charged particles with negative field weights, but that didn’t work; merely acted like the field weight was zero.
What can I do?

How much tricky stuff involving code compiling would there be if I decided to add in the extra forcefield as a mod?

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