Like @jpfieber, I also have some reservations about doing something lossy with the data, but, if it were me, I would:
- Make a copy of the Balance History sheet in a new spreadsheet
- Use filters to delete everything but the accounts I wanted to “merge”
- Use the Trim Balance History workflow in the Tiller Community Solutions Add-on to reduce the data down to one balance record per month
- Depending on number of rows remaining, either manually combine balances month by month leaving all the other fields for one of the two accounts… or do something clever
- Delete the merged accounts out of your original spreadsheet
- Paste the merged data back into your original spreadsheet
- Sort your updated Balance History sheet