Is it bad form to be a member of more than one cc?

As both clubs will be benefitting from your subs. it shouldn't be seen as a problem by either one, unless one has "issues" with the other, which might tend to alienate you from a section of each club.
As has been mentioned, if racing you must declare one of them to be your first claim club, even then it's not set in stone, as long as you advise all interested parties.
On this subject I'd like to pay tribute to the long-time TT champion and LE-JoG record breaker John Woodburn who joined my own (then sponsored) club in 1981 and helped to bring us much success. From the gallery of high-profile star riders who sought pastures new when the club sponsorship ceased, John was almost alone in retaining his membership.
It's accepted that John has competed for various clubs since then but he has remained a loyal 'second claim' member (and now a life member) even though he lives almost 200 miles from our home base.

Nothing wrong with it.

However, it is Not The Done Thing to try to either wear the kit of one local club at another's ride, or try to recruit members of one club for another.

Do let your first club know though, in case they see you out in a different kit.

If you are guesting at another club not within spitting distance, it's fine to wear your original club's kit.

My club does an affiliate membership.

I live and work 150 miles apart so I have a weekday and a weekend club, I like to wind people up by wearing the 'wrong' kit sometimes

It's fine

As far as I recall it all comes from the days of proper sports clubs, and the whole 'First claim' 'second claim' thing is to prevent say, Sir Wiggo, turning up, joining every club within 50 miles of Chorley and winning every club championship then taking home 16 crates of beer, three sheep, a black pudding, two hundredweight of coal, three horseshoes, a pair of socks, a pair of clogs, two new collars, a waistcoat, two tin buckets, and a new string vest.

It's not unusual at all. When you join a BC club you have to nominate your "First Claim" club - in other words, it is accepted that you might also be in another club.

I don't see it being a problem, unless you are racing, in which case you might have problems with affiliation etc. If it's just club rides you do, then who really cares if you go out on occasion with a different group of people.

I don't personally see a problem, although I suppose you might want to avoid wearing the 'wrong' club kit when out for a group ride.

We have a few members who live a bit further away or even just work the wrong shifts and can't currently ride with us as much as they used to, and they are in other clubs as well as ours in order to ensure they can get onto group rides that suit them.