According to the book 'Keeping Australian Reptiles in Captivity' the minimum enclosure size recommended for 2 adult stimsons is 600x400x400. Personally I think that as long as they are both less than 1M this will be okay, and as you have 2 females they shouldn't ever fight. Just be careful how you feed them, 1 at a time, and you should be fine.
That being said, I think a bigger enclosure (not enormous) couldn't hurt, just observe them and decide for yourself.