I am event partner and had an adult coach ask me the following question:
Is it legal for a junior high student to compete in VEX IQ and VRC during the same season? I said to my knowledge that was legal and that the only rule I was aware of was that they could not be a driver for multiple teams at the same competition. In this particular case, three junior high students participated in a VEX IQ competition that was three weeks prior to the VRC competition. Personally, I was impressed that they built robots for both competitions.
There are no rules I know of which would prevent it. Last year the facility where I mentor hosted a simultaneous VEX IQ and VRC event. Even then, the participants you describe could have driven in each competition, since they are different events that just happened to be hosted in the same facility on the same date. As you point out, there’s no rule against this.