"Rob
All you have to do is to keep them at the same level or lower as your main teams level. So I trained alot in Physical, as that gives them the least leveling.
This.. I have made three active accounts myself that are all level 11, one about to hit level 12. This makes it handy for me as far as drivers go etc.
I've also recently been gifted two level 12 accounts by a friend who no longer plays. Currently I haven't done anything with his accounts as he already had good young drivers on there with plenty of talent etc but it's handy to keep the levels up etc and they are still racing in their respective leagues.
Personally I don't see the problem with multiple accounts - So long as you either a) Have them in different leagues and/or b) If they are in the same league together and you aren't using them to hold anyone up, there is no real issue.