Is it permissible to split the cost of paid medical references with a colleague for mutual benefit, or must each individual pay the full amount separately?

If the questioner and her colleague will co-own the references and benefit from them alternately, then there is no objection to that. However, if they will buy the references and then make copies from them, this is not permissible according to the prevailing fatwa which considers copyright absolute, and the more cautious approach is not to copy.