Added to the latest version (21.5.40)
There are some changes I need to go through.
Bank rules are no longer called bank rules. Instead, Receipts tab has its own Receipt Rules and Payments tab has its own Payment Rules. This will allow to create separate set of rules for each transaction type.
In order to access your list of Receipt Rules, go to Receipts tab then click on Receipt Rules in bottom-right corner.
When you have uncategorized receipts, you will see an alert at the top of Receipts tab screen which will prompt you to categorize your uncategorized receipts using receipt rules.
When you create new (or editing existing) receipt rule, there is new button Add Line on the form.
Clicking this button will allow you to define split rule.
For example, you can define receipt rule which will split receipt in two equal line items
However, you can change the Percentage to Fixed amount on any line item to specify fixed amount.
The way splitting works is that Manager will first allocate exact amounts, then reminder will be split by percentage. The rules support negative amounts too.