I'd say it is the concentration of Test in the oil. What carrier oil is used? Some oils that can barely handle 400mg/mL will cause more discomfort than an oil that handles more.

From experience of mixing different injectables, I would just pull the required volume from separate vials into one syringe. You just need to keep in mind where to stop pulling the plunger.
I've pulled Test E, Tren A and Deca all in one syringe and pinned it in my quad with no issues.

Although, mixing into a single vial is popular. Test P/Tren A is done a lot. My source mixes a Test P/Tren A/Mast combo at 100mg each, its really popular.