mixing primer with paint for first coat on exterior concrete walls?

we are currently getting quotes to paint the exterior of our house.

One painter has suggested that his paint supplier would pre mix primer into the first coat (i don't have details yet, but going to the paint supplier later today)

Is this an option - I would have thought that primer should be applied to the concrete first, let dry and then start the paint job? But maybe there are products now that can be used to mix with the paint....