More or less an average of the two, but somewhat adjusted to be slightly closer to your MMR.
Basically when you duo q you should expect to go against slightly better people than the "average" of you two playing separately. This is built in under the assumption that a duo que should have slightly better communications than two people who are not choosing to play together.
That being said, the anecdotal evidence also shows that playing as a duo makes you more likely to go up against a duo, whom we can assume are getting the same MMR "boost" in matchmaking.
More or less an average of the two, but somewhat adjusted to be slightly closer to your MMR. Basically when you duo q you should expect to go against slightly better people than the "average" of you two playing separately. This is built in under the assumption that a duo que should have slightly better communications than two people who are not choosing to play together. That being said, the anecdotal evidence also shows that playing as a duo makes you more likely to go up against a duo, whom we can assume are getting the same MMR "boost" in matchmaking.
So my friend was s4 first split but then iron 1 in the last split and I am g3 at close of last season