Splitting publication fees among coauthors
I think your question is a little backwards: All ways of splitting the publication fees are acceptable. You might run into some problems with your grant officer about transferring money around, but in the end they will get over it.

The more important question in my opinion is: What behaviour is acceptable from you and your co-authors. I believe the answer is to be nice since these are your co-authors after all. I wouldn't be too put off if a co-author refused to contribute to open access fees or if I was asked to contribute 1/N of the fees for an N author paper where I was a tertiary author and currently under-funded.