Can I use a same brush for primer and paint
You can use the same brush with several types of paint in a short day.
Pro tip from a professional:
Thoroughly wet the brush with water. ( doesn’t need to be hot or warm.) give it a few shakes, tape the medal part on your shoe or tree, wipe it off back and fourth aggressively on your pants or cloth.
This prevents the Paint from being pushed so deep into the fibers.
Paint as you wish
Wash it right away
And repeat.
This DOES NOT dilute the paint whats so ever.
Sometimes when I’m painting commercial jobs or any type of paint that ends up drying more quick than I would like it to after an hour or so I will wash it clean and keep going.
Using a brush without wetting it is referred to applying with a dry brush which is not a normal application.
Some people disagree with this as they believe it will increase the chance of leaving a halo mark at the first 4 inches at the top of the wall Due to the fact you will be using a dry roller this does not hold true.  If a painter is getting a hail at the cut in line then they are not stippling it with a 4 inch roller.  When an artist paints a picture they continue to use the same brush for several colors with no problems what so ever.
Hope this helps !