Not only will it make your asthma worse, but you'll probably find
you're getting sick a lot more often.  Why would you do that to
yourself?  There's no way you can keep a house clean enough to
avoid having your condition worsen.  Even with the no cat in the
bedroom rule, you'll carry the dander/hair in with you on your
clothing, and by the time you go to bed, you will have been
exposed long enough for you to probably have an attack anyway.

Sorry, but I think being sensible means just saying no to the
cat.

You're crazy.  The problem is, your asthma isn't very bad at the moment
and you don't realise how bad it can get.  Thank your lucky stars that
it's mild at the moment and whatever you do don't do anything to
aggravate it.  You don't really want to end up on oral steroids
and being regularly hospitalised, do you?  So, I'm afraid you need
to say no to a second cat and I can only say that your most sensible
course of action would be to give the one you have now to another
home that does not have asthma.  Asthma is a serious illness and
you need to take it seriously.  Just because it's mild now doesn't
mean it can't get worse - a lot worse.

Tony