4x2x2 is a boa sized cage. Really, I don't think anything bigger than a 20gallon or a 30 gallon is healthy for the snakes.
Just because something hasn't happened, doesn't mean that it rules out potential issues of keeping the two snakes together. If one gets ill, they both get ill. Cannibalization is possible, and if they are two males, they could fight. Male and female could breed, and if she breeds when she is too small, cause egg impaction, which could kill the female.
Just overall, a bad idea.
No matter what, to heat the enclosures, you must use a good thermostat with the probe taped directly on the tape or UTH.
Ceramic heat emitters are also another option.