Would injecting Primo and Winny together in one shot make a difference or should they be done seperately?
If it’s winny in a water base, it won’t mix with the primo, but you can still inject them together without any problem.
JP
Just remember oil is more viscous(sp)than water. If the oil and waterbase are separated in the syringe and oil goes in first, ease of the pressure on the plunger or you’re in for a nasty surprise. PAIN!! Trust me, I know 
On a similar note, I used to mix my Test Prop and Winny in the same syringe during one of my cutting cycles. All I did was pull 1cc of each into a 3cc barrel, and pulled 1cc of air into the barrel as well. Then I just shook the barrel for a good 10 seconds to temporarily “suspend” the Winny in the oil. Then all you do is purge that 1cc of air out of your barrel and inject.
Like Moose said, the viscosity of both solutions are different. The pressure you would have to apply to the plunger is different between oil and water. The injections will be alot smoother if you temporarily mix the two. Good luck.