A 4x2x2 would work well for an adult dumerils. I agree with Pythonis...don't give your dumerils a roomate! If you want another dumerils than cage it separatly.
I have 30 snakes and 5 more in eggs, plus a gravid female with ? eggs, so I know what it is like to be short on space. Dumerils can be cannabalistic, but also, stressed easily! I have tried to house ball pythons (who are not cannabalistic) with other balls in every gender pair imaginable. It always starts well, but with in a few months one or both of the snakes is off feed. Snakes have to compete for a hidebox, a hot spot, etc. A lot of Kenyan sand boa breeders keep these boas in a 1:2 ratio year long for breeding. I kept my trio together, and no one will touch food! So, now they are separate and eating well. What works for some, isn't always best.
At the very lest if the above does not concern you I would suggest you look for a tank that is 6x4x4 or 8x4x4 with a possible divider in the center, so you can separate the dumerils when issues occur. Also, a female dumerils is gravid for 6-8 months, I imagine it would be extremely stressfull for her to have a male present constantly during that time.
Here is what I did:

Each is 4x2x2..works perfect except I need to add locks to the plexiglass! The top two cages each house a 6 foot female, while the bottom cage houses my 5 foot male.
Good luck!
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6.14 Normal ball pythons
1.0 Pastel ball python
5 normal x pastel eggs incubating!
1.0 Plains Garter
0.1 Rosy boa
0.1 Normal Kenyan Sand Boa
1.1 Anery Kenyan Sand Boa
0.1 Leucistic Texas Rat
1.2 Dumeril's Boa
-----My list is too long, so I'll stop here!
*Amy*