I suggest doing it separately.
I also suggest immediately telling him you can't be best man. Lie if you need to.
You don't want to mangle his wedding day. And you being a best man that in time ends up being a farce in his eyes, well, you will have ruined the memory for both of you.
So yeah, I'd step down as best man before the weekend is done actually. And a lie will do less damage than his 'best man' not even being a man.
Forget what family says about the wedding stuff, the bride and groom will appreciate their wedding photos not being a total waste.
Dear, it would be nice if you could be a bridesmaid of course. But that is likely asking for too much. But life will be tricky enough, avoid anything you can avoid that can only go ugly.
I myself was a 'best man' once. I don't know what he thinks of the pictures these days. I don't care though, he ditched me. I was also a husband once. Yeah, I don't want any of those pictures either. I have long ago gotten rid of all evidence he ever existed. You only want to be making future happy memories.
I dream of someday being a bride in a white wedding dress. I dream of all THOSE possible happy moments.
I think it would be funny if an old friend was a bridesmaid in my wedding again

This time she would be MY bridesmaid, not the ex-wife's bridesmaid
