worse case senerio youll need two coats try it and see ive done the same with mixed results normally it takes two coats to get adequate coverage but it really depends on the products your using

Should be fine.

I'm painting my living room and I am painting over a previous colour. I bought some paint the other day and, without thinking, I bought a paint mixed with primer (I was painting a previously unpainted bedroom before that, so that required primer and I had it in my head)

I'm going to need another gallon for sure, but I don't want to pay extra for the primer. Would there be a noticeable difference if I used a paint+primer combo for one layer and then just plain paint for the second layer.