I use a bow, and when I want to dualwield sword+dagger, the game removes my dagger from offhand.
Is that a bug? what wrong I do?

Pair the dagger with the sword so that it automatically equips when you switch back; you can also do this with other types of offhand equipment (pairing that bow you're using with a quiver might not be a bad idea).

That's what I do, but switching from bow+quiver to sword+dagger removes the paired dagger.
So, I rather use a 2-hands sword, there is not this problem...
Or another bad solution is to abandon the bow, I keep dualwielding with dagger and never switch. :/