Iam et Marcus et Quintus mala habent.
A comment by TKR pointed out that in addition to the question of whether "malum" should be used as a distributive singular, there's also the question of what the number of the verb should be.
According to Harm Pinkster (The Oxford Latin Syntax), an emphatic coordination (et...et) usually involves the verb agreeing in number with the nearest subject (so being singular if the nearest subject is singular).
For example: et proavus et avus praetor fuit, "both the great-grandfather and the grandfather were praetors" (literally "was a praetor").
Pinkster mentions that there are "also instances" where writers use a plural verb though, as they normally would for a straightforward "et" coordinated subject.