i done 10 teams in serie a now but have jus read this so i dunno whether u would want to risk your hard work?
How to merge multiple data files
This is a complete nightmare, and you should avoid merging data at all cost. Whenever you have newly created data in more than one file and want to merge the two, things go absolutely haywire, and you won't even always notice it right away. There seems to be a bug in the Merge function that doesn't properly reassign IDs to newly created objects in the second file, so you end up with duplicate unique IDs for different objects, causing teams, players, stadiums, etc to outright disappear. So again, simply don't do it is by far your best option. However if you must, and you're into masochism, there's a way to salvage hard work when it's split up in multiple files.
Again this involves editing the XML files manually, by using a text editor. The idea is to give all newly created objects in the second file new IDs, and getting rid of all duplicate data, such as competitions that are created in both files. To find the duplicate data, I recommend using winmerge, a freeware utility that allows you to see differences in two files side by side. Delete all the duplicate data in the second file. There will be some minor differences because the editor tends to change datatypes from "unsigned" to "string" for no particular reason, but you can ignore those. It will work fine regardless of datatype.
After you've weeded out the duplicates, it's time to reassign IDs. I can't get into great detail here because this would be a novel, but if you figure out how new objects are stored in the XML, you can identify its unique ID and then do a "replace all" to replace it with something that's larger than the highest ID in the original file. Do that for each object, be it a club, person, competition, city or stadium. When you merge the modified data the editor will properly reassign all IDs to be in sequence.
Hopefully SI will fix this in the next patch, but for now maybe this saves some people from having to throw away a lot of work they've put in.