No need to stack!
Personally, I would never do this, and nor do I recommend it. But, since you are hell-bent on going forward, here ya' go. But she better not come whining (in a deep baratone voice prolly) about being screwed up with sides. You have been warned by some VERY knowledgable people above.
IF she had NO sides from her last deca only cycle, then start with half the last dose for 15-21 days. If she still has no sides, use the remaining deca in EXACTLY the same manner that she used it successfully before. Then, when that is OUT OF HER SYSTEM COMPLETELY, then start on the Primo. No need to worry about losing any gains, she won't be off that long between. BE aware how long Deca stays active in the system though!
Because deca is a LOOOOOOOONNNNNNNNGGGGGGG lasting AS, she needs to be especially cautious to NOT have much in the way of residual deca still active when she starts the Primo. If she develops sides from this combo, be aware that they will probably be VERY prominant and perhaps irreversably PERMANENT due to the length of time it will take the Deca to clear. If the loss of singing voice, deep voice, extra (maybe coarse) facial and body hair, loss of head hair, wacked endocrine system, etc. are okay; then go for it.
Consider that Deca will be released from the depot (intramuscular injection point) at a steady state for about 4-5 days, then begin to taper off. The peak blood serum concentrations of deca are usually seen in about 3-6 days. Deca has a major affinity for fats and is very easily taken up by the cells. In persons with high fat concentrations, deca will stick around for a loooong time. Hence the greater than 6 month detection times for nandrolone. I would wait for 9-12 weeks before using another androgen.
In short, be overly cautious.