It's the bare minimum acceptable size for two females, males would definitely need more space. Seeing as you didn't purchase any cages yet, I'd look at C&C. This cage, for example, is much more appropriate for two girls (still too small for boys), and it's not much more than the midwest on EBay.
Normal 2x4, no lid or loft, $63 w/ $20 shipping
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Normal 2x4, no lid or loft, $71 w/ $15 shipping
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2x4 covered cage (you'll only need this if you have bigger animals that you're afraid will get to the pig, or small children. Guinea pigs aren't the best escape artists, so they don't need a cover. The covers are more for protection)
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Anyways, $107 with $17 shipping
2x4 with a 1x2 loft, $69 w/ $19 shipping
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And that's just on the first page of EBay
You can also make your own C&C if you're handy... you can buy the grids nearly anywhere (Bed Beth and Beyond, Amazon, Home Depot, the Dollar Store), the coroplast is only $13 for a large sheet at HomeDepot and they're held together by zip ties.