I usually carry on two bags on all flights (Air Asia, or otherwise). Never had a real problem, but occasionally on other airlines, they'll say the carry on is too heavy. Only at this point, do I bring up the cpap issue.

I've had no problems in the 4 years or so I've flown Air Asia with carry ons. Last problem with flying intra South Africa on LCC Kulula. They made me put a checked tag on it, since they were unsure if I could take it on or not, but I took it on the plane, anyway, with no problems.