Say, the situation is intervocalic., for example:
[ata] > [aða]
The differences of the two sounds are whether the tongue touches the palate and voicing.
So if I assume devoicingvoicing happens first, the problem becomes thus that in some languages do voiced plosives not exist, so it is implausible to suppose this step happens first. If I assume the other one happens first, it's also implausible since it would ben aan unvoiced [ð̥].
So how would such a sound change exactly happen? Can they take place the same time?