The situation is:
On FM2008 which I was playing till recently, you can have a Club job, still apply for a national job without the usual (fans kicking off). Then if you are offered the national job you can take it, AND keep the club job with noone complaining (WHICH is a complete glitch because you can't in rl have a club job and a national job at the same position, thus why Steve McClaren could be Asst Man for England and Manager at Middlesborough and why he had to quit Middlesborough when he got the manager's job). In FM2010 I haven't been offered a national job that I wanted to take, but i apply to every one that opens, and I was offered the Martinique job.
SO it is UP TO YOU... you can have both jobs, and break the rl rules, and noone gives a hoot, or you can pertain to the rules as I am doing at the moment, and Leave your current league job when a good nation opens up. ONE MAJOR POINT, if you get a good nation, your reputation will rise, i was a national once with Fulham, got the England Job, did well in the Euros and got Continental, then won the next European and recieved World Class status....