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Asthma is pretty rare in 4 legged animals and it has to be diagnosed by the vet using x-ray. If you have not done so, there's a good chance that it's not asthma at all.
I would not use any inhalers without the advice of my vet.
If you suspect that there is a respiratory problem that they both might be suffering from, I suggest that you see the vet to get them started on antibiotics. Sometimes one good round can clear it up.
This breed is prone to allergies, so you might be seeing the effects of THAT. If so, your vet can prescribe an anithistamine that will give them some relief.
I hope this helps!
Let me know if you still have questions.
Because it comes and goes, I really think that you are dealing with an allergy problem rather than asthma but, by all means, you should explore this further with your vet.
Keep in mind that heart problems can also cause coughing and breathing difficulties. I would be more worried about this if the cats are not on heartworm prevention.