A lot of runners are members of more than one club, i'm a member of my local club and the rrc, mostly i run in my local club colours but will run in my rrc kit when competing in one of their championship races.
You really only really need to nominate your first club if your competing in club competitiions such as cross countries, hope this helps.
if you are a member of two clubs one has to be the first claim, which means that in a race you have to run for that club if both clubs have runners in that race. however if only your second claim team has runners (usually this will be a league race which is restricted to the entered club teams) then you run for them. we have had several members who run for us in leagues and socially but run for a bigger local club first claim so that they can do national events.