Most people track dumbbells as 1. You enter the weight of each individual dumbbell as the "weight" even though you lifted two. This is the same regardless of whether you do a two-arm bilateral move like a chest press or a single arm unilateral move like a single bicep curl.
But ultimately, as Heather said, the value entered doesn't change anything. So just do it any way that makes sense to you, because all it's there for is for you to look back over time and see how you did. So it only has to make sense to you.