That depends on the mechanics of how A controls B for which we have multiple possibilities based on what you specified:
- Mind Manipulation which would be hax.
- Body Puppetry which in many cases is also hax.
- B is a robot and A uses a remote to control them which wouldn't lead into scaling.
- A can give B orders which B feels he cannot disobey which does leave some ambiguity for the scaling or the cause.
- A uses a power to control B that requires the user to be comparable or stronger than the target.