I'd like to give feedback on this but the description is very thin for useful feedback. On the whole I would try to avoid a situation like this though.
Was it on the same stoppage (I assume so)?
When you say separately, do you mean at different times and in sequence?
Was the second dissent after you had shown the first yellow, (how bout the next sequence)?
How did you deliver the warning and each of the cards (I know most of these are YHTBT and hard to describe in words)?
From a game/player management view point, dissent yellow card is used not so much for punishment but to curb behavior for the better (sin-bin was introduced to stress this point). Sometimes you need to balance the deserved punishment with the negative impact it could have on curbing poor behavior.