Is it really necarssary if keeping just 2 females? I am more inclined to get females now as I really don't want my bedroom to smell after the males have scent marked their cage, but I can't afford to get females spayed.
Spaying is your decision. Although, there are many health benefits that are created by spaying, for example it significantly decreases the risk of mammary tumours (and pituitary tumours, I believe, not sure though) between 75-85% if they are spayed young.
However, not all people spay their girls - all of mine are in tact and I'm inclined to keep them that way as I've recently found out there are no vets in my area experienced in spaying rats.
The question you may want to ask yourself now, especially if money is an issue, is that will you have the money later on to pay for any tumour removal operations they may (just may) need? Perhaps scrimping and saving now will save you additional bills in the long run? Just a thought