In theory, ideally you'd choose one or the other, to get all of the compounds dosed properly. In actual execution, the difference is probably small enough that if your brother likes some aspects of one and some aspects of the other, he could do that. I'd probably start with 2 caps of Ignit3 though because otherwise you'll be getting a lot of stims, a lot of rauw, etc.
However, if he likes the energy and mood boost from BSR and wants the more multi-faceted approach to fat loss from Ignit3, I'd probably stack BSR + Assass1nate instead. That way you can get full doses all involved compounds, get the energy and mood benefits from BSR, and hit fat loss through several pathways via Assass1nate.