IMO take 1/2 scoop of Assault and 1/2 scoop BETA-CRET if you want to mix them. Over half of the ingredients in BETA-CRET are in Assault. Both are proprietary blends so its impossible to say at what point your taking too much of something.
Not too good stack IMO, Assault by itself is very good but if you want to add something I would recommend SNS Agmatine or iForce Hemavol.
EDIT: I found this
So adding BETA-CRET to 1 scoop is not a good idea.