The dangers that might present from this sort of combination are going to come from the potential intensity of the experience, not from any sort of dangerous reaction. Be aware of how intense such a combination could be and make sure you're using significantly less of each compound than you'd use when taking them on their own.
I personally found 150mg 6-APB + 9mg 4-AcO-DMT to be a lovely time. In future I'd take less 6-APB though, and maybe a tad more 4-AcO-DMT, your 6-APB and 4-AcO-DMT dosage suggestions sound good to me.