I know people that keep bloods together and have done for years with no detrimental effects although in general it is normally only for breeding purposes.

Certainly you would need a very large tank to house 2 bloods so they are not immediately competing for space or privacy.

I have had two of my bloods together in a very large tank for periods while I was renovating new tanks etc with no problems.

Personally I always advise to house separately, but as I said I do know very experienced and capable breeders who have done and do keep pairs of bloods together but there is double of everything to accommodate this and they have healthy breeding bloods.