From time to time, we have contributions that come in which need to be credited to two accounts. How do I split a contribution?
You could create two contributions, as @ErikH mentioned in a comment.
You could create a price set with two fields configured for the two accounts. Then create a contribution using the price set and have two line items, one for each account.
FYI, we expect with a month or so to have a new version of the Line Item Edit extension which will allow you to easily add an additional line item without first defining a priceset. That will allow you to split the contribution between accounts more easily. See https://github.com/JMAConsulting/biz.jmaconsulting.lineitemedit/wiki
CiviCRM doesn't allows splitting of contributions out of the box. However if you are a developer or if you have access to developer resources you could probably develop an extension which does what you need.