i wouldn't be handing over DD's birth certificate for something like that - there is no need for her to have multiple accounts - she is a minor, you're co-signing with your partner (can't call him a DP given his ultimatum) - it's not like you're going to bleed her dry. if what they are trying to do is to start a trust fund for her that is tied up so that she can't access it until adulthood, then maybe there is a call for another account - but they don't need to take her birth certificate - they can ask you to accompany them.

if your partner is prepared to leave you over this, i would have big questions about your relationship - and i would be putting everything you can in place as a contingency - bank account details including balances etc. make sure DD's birth certificate is put somewhere safe. it's a pathetic thing to play with your relationship like this. he sounds rather immature. your DD is YOUR DD - she is not the child of his parents - and if one of you isn't comfortable handing over very important proof of identification, he should respect that. there is a reason they only give copies of birth certificates to the informant!