no middle man needed. just each other being co-signers to make sure it only leaves the multisig when both sides are happy with destination and amount and time to let it be spent
in short you both become your own escrow
It's only work if both of them don't have any problem, but if one of them is a scammer, the other ones will become a victim because he has no way to get back his coins. Unlike having third party intermediary who can evaluate and judge which one is wrong and right.
But i don't get the point at all about what are you trying to do, if this process is for building trust then you don't need the escrow because that breaks the idea about building trust between your partner and you.
As big as you trust someone, trust no one. This is about money, probably you think $400-$900 is not much i.e. you can afford to lose, but in many countries it's worth like 6 months salary.