5 gallons is PLENTY for a single betta....the "divided ten" is one of the standard enclosures most betta owners use. i know some people will divide a ten into two 2.5's with a 5g in the middle, with a plakat in the 5, and long-fins on either side. it's beautiful, but i'd be worried keeping them that way...too many dangerous variables at play.
So remember, when you're feeling very small and insecure,
How amazingly unlikely is your birth,
And pray that there's intelligent life somewhere up in space,
'Cause there's bugger-all down here on Earth.
Loki/Grendel/Gail/Beowulf/Camilla/River/Bathory/Arwen/Lafayette/Deb/Jack Skellington (soon)