If the question is being asked about a single donor, then you can run the Gift History report and exclude gifts made to the donor's fund to get the question answered. The difficulty is when the question is being asked for all donors and you want a single report to show the information. Fortunately, you can modify codes on gifts even if they have been posted without doing a formal gift adjustment.
To best answer this, we suggest adding a different gift purpose code to a gift to manage this. Most foundations have addition to fund - ADD as a purpose code. If you create another code (e.g. ADF) , then you can have ADD as Addition to the Donor's Fund and ADF could be Addition to another Fund. That way ADD thru ADF is still addition to A fund, but by selecting just ADF, you can now get to the answer to the "How much are donors giving to funds other than their own fund?" question.