"Merging" Two quicken files

Since there are no transactions using the mileage category, it doesn't matter if it exists in one file but not the other.

Not necessary, Quicken will create a new category if one exists in the qif file but not in the file being imported to.

Actually though, I think I would head in the opposite direction.

Since neither of you use the mileage category, I'd think you would be happier if you ended up without that category in your final file. And exporting to a qif file will give you the opportunity to get rid of that category, since you can delete it from the qif file so it won't be imported into your final file.

If you open the exported qif file that contains your categories in Notepad, near the end of the category records you should find something like this:

N_MileageBus DBusiness Mileage T R1320 E ^

I think "_MileageBus" is the first mileage category and "_MileageUnreimBus" is the last mileage category. You should be able to delete all the _Mileage... category records at once.

Delete each record starting with the one with the "N" in column one (the one with the category name) through the one with the caret ("^"). Do that for each "_Mileage..." category in the file, then Save the file. (Don't worry if the records between the first and last for a category look different than what I posted, or if there are more records between the first and last than my example; just delete everything from the one with the "N" in the first postition to the one with the "^" in the first postition ... for each category you don't want.)