Entries Already on a Target Matter when Using Orion's Split Billing Feature?

Yes, Orion's split billing feature is simply going to present the bill hours and  amounts and values before the split along with a line that says "amounts due from other parties." If there are already entries on the target Matters,  it would include those in the total as well.

If your split bill style has any sort of percentage calculation showing, this would be affected if additional entries have been added to the target Matter and could be different from the "original" percentage you initially set up on the split. This may be fine; however, you should examine the Invoice carefully to make sure it presents what you want to show.  Typically the split bill does not show the actual percentage amount on it, but it could, depending on the Invoice Style.