Can somebody explain me how OPML works for RSS? Are these files usually imported into the RSS reader apps or are they used where they are? If I import multiple OPML files with multiple feeds, will the feeds from the first OPML be overwritten by those in the second one or will they add up? Will article read/unread status be synced to multiple devices if I use the same OPML file or not?
As n OPML file just contains the definition of which feeds you want to follow, not the actual RSS data. They're handy when you want to switch from one RSS reader to another. You can usually export the settings from your old service and import them into the new.
Check with your RSS service but importing an OPML file should always add new services to the list, not overwrite.
I don't think an OPML file tracks what you've read or not but I could be wrong about that.
OPML files really aren't much more than a list of the feeds you're subscribed to. Individual posts or articles aren't in there. I would expect that importing a second OPML file would just add more subscriptions, but it'd be up to the reader app to decide what it does.