Not sure if they have these in the UK, but I keep two females in the 18" x 18" x 24" version of this tank and have had no issues. The key is to keep a lot of plants, hiding spots, branches, etc. so that they can have obstructed lines of sight and also to provide multiple food bowls so that one can't chase the other off.

With that tank and the similar exo-terras, you can keep live plants as well which helps with humidity. I use this setup in my classroom (they are my class pets) and it looks great.

With the geckos, just make sure the two are the same size / weight and also introduce them both at the same time so that no one has "claimed" the tank as their territory. Mine scuffled for about a week or so to work out the pecking order, then they were fine. It's always recommended to also keep an eye on their weight when housing together to make sure neither is stressing out or being bullied. Just be prepared to pull either if things don't work out.