Primer for my drywall project
I think the best bet is to use up the remaining primer in the pail by applying it to the drywall. Then start with the new pail applying on the remaining drywall. 

Even better would be to let the residual area dry before coming with an overlapping coat of the new primer. 

Some paints would mix just fine whilst others may not. So why take the chance when the solution is simple.