There are 2 ways you can proceeds.  One way - which comes with some safeties - is to start a second 'employee' file in Sage for this person and do two adjustment paycheques - a negative payroll in one payroll account and a positive payroll in the other payroll account.  The other way is to manually edit the .xml file.

If you decide to edit the .xml file:

First, be really, really sure you know what numbers you want where.  If you manually adjust the .xml file, there are no safeties of any sort.

Second, be aware that CRA pretty much ignores the numbers you put on your T4 summary, and just adds up all the CPP/EI/Tax deductions shown on the individual T4's in the .xml file.  However, you still have to have accurate T4 summary (or for 2 BN's, summaries) in your .xml file.

Third, CRA is fine with you uploading one .xml file for your first BN, and then a 2nd completely different .xml file your your second BN.  In other words, you can upload more than one .xml file relating to a single year of T4's.

Then, if you're sure you want to edit the .xml file, here are the steps:

1. Create the file as usual in Sage.

2. Open the file using a web browser that renders .xml files in a nice, readable format.  I've found Microsoft Edge to be good for this.  You want a format that automatically does various levels of tabs, automatically colours different elements differently, and allows you to expand and collapse sections.  You should see various down-arrow icons (for example  at the start of the T619 section [these down arrows should let you collapse the section they point to], and a bunch of section start markers () paired with section end markers ().  Do NOT use the Office XML handler, as I have had really bad results with it.

3.  Find the <T4Slip> or slips you need to edit, and make changes as needed.  If you need to create a 2nd <T4Slip> under a different BN for this employee, you should be able to copy and paste.

4.  Double check the totals in your <T4Summary> section or (for 2 BN's) sections, especially the # of slips.

5.  Upload the file to CRA as usual.

Thank you! 

Further note.  I just finished manually editing one of the .xml files that I always have to edit - to change province of employment to ZZ because Sage doesn't support this.  I found that I was no longer able to edit the .xml file in any of the nice, easy-to-read web browsers I tried.  Instead, I had to do the edits in hard-to-read Notepad (making sure to leave the extension as .xml and not accidentally change it to .txt), and then open it in a web browser so I could read it over and make sure I hadn't loused anything up.