Items can be split across multiple projects using an option in the branch profile.

Please note that only one of the "Allow PO Items to be split" options will be allowed to be selected as it is an either/or option.

If selecting projects by percentage, you will need to select the option under project when creating a transaction:

Next step will be to click on any of the add items option:

You will now have the option to choose how you want to split the percentage for that item, when clicking on split, options to split across projects will become available: