I think you should verify that both SQL servers are running the same sort order before planning your merger. (Query:sp_helpsort (in text mode)). If they are not the same sort order you have a much bigger task ahead.
Also, you don't indicate whether you are on BRL or module basis - the systems both should have the same selection of additional dictionaries (and third parties).
I would upgrade v8 to v9 first. Create the v8 companies in the existing v9 system and do backups from the upgraded system. Restore these into the existing v9 system. Resolve user issues and note problems. Upgrade then to v10. Need I mention backup extensively?