Is it okay to discuss multiple interests when writing statement of purpose for attending a conference?
Keep it short and succinct, I would say a paragraph is good, unless otherwise stated in the application. If this means that you only have enough space to talk about one of those areas, then pick one.

If the conference is more geared towards pure mathematics, I would suggest expanding on your interest in cryptography. Otherwise, if it is an applied mathematics conference, then talk about your studies in biological math. Chances are schools which specialize in pure mathematics will attend a pure mathematics conference, and likewise for applied math. If you want to find out the universities who are attending, just ask an event coordinator. I'm sure there isn't any secrecy surrounding who is attending.