I recently purchased gamer club last week, but I completely underestimated my sales and I already used up the 50$ limit in 2 days. I want to buy pro club, so I can have a 500$ sale cap, but will I have to wait until my gamer club expires for the pro club benefits to start taking effect?
Answering your topic title, yes. You can have more than 1 active.
No, you do not have to wait. Since you already used up Gamer Club, if you have Pro Club as well, it will use Pro Club.