Its based off solo que.
The tier ranking for clash is correct for you as of now.
What happens is this.
It is ranking you overall vs everyone else that has already joined a team for clash.
What this means is as more higher mmr player join teams your overall tier will drop.
Again stupid way to do it but just how it is.
Right now you and your friend are in higher tiers cuz more lower mmr players have signed up then higher mmr players.
so you are currently actually in the highest 2 tiers compared to everyone else signed up.
Does that make sense?
Your tier in clash is based off not only your solo que mmr but also the opponents that have signed up to play as well.
If no one higher mmr then you signs up you will remain in your current clash tier.