You will rarely get people on this site telling you NOT to get another betta
How large is this divided tank?
I had my HM and my super delta bettas in a divided 10 gallon. My HM started tail biting because seeing another betta stressed him out. My super delta was tail biting before I put him in the divided 10 gallon. Once I separated them (Super delta in the 10 gallon, HM in the 3 gallon), they stopped tail biting.
Moral of the story: Be prepared with another tank in case things don't work out with a divided.
I am going to try a divided 10 gallon again with two different fish and see how it goes, with another tank on stand by in case things don't work again.
There's always more room for bettas

And the extra money you save on a betta can go towards a new tank

It's good to have a spare tank handy anyways in case one betta becomes ill and you need to treat him separately.