I have two females in a 9 gallon... I just couldn't resist their cute little faces at walmart. It took them about 2 hours to establish dominance, and now they're swimming around together and being the best of chums. When my male who's in the 30 gallon departs, I'm going to make that the female tank and porbably add four more females, and Divide the 9 gallon into a two or three sectioned tank for males.
so, back to your question: If you wanted to put them in now watch them close and make sure they adjust to each other... then, when you buy more on the weekend you'll have to take the two out and re-arange the tank then put them in 'new' tank all at the same time, and wait for dominance to be established again,
or,
if you don't want to have to worry about taking them back out and re-aranging, it would be best to leave them seperate. Its up to you on which way you do it.