IIRC, that was true in 5e, removed in 6e.
Edit: Found it! 5erp311, "However, a character may not combine two or more slots from a single Power Framework as part of a multiple-power attack, even if he has sufficient reserve or base points to use both slots at once."
6e1p399, "A character can make a Multiple Attack (6E2 73) with two or more slots in a single Power Framework, provided he has enough reserve/Pool points to allocate to the two or more slots used in the attack simultaneously. A character can use slots from two or more different Power Frame - works to perform a Multiple Attack, assuming he obeys all the rules for such attacks." So it looks like that's already a Multipe Attack, not a Combined Attack, by RAW.