Is it okay to discuss multiple interests when writing statement of purpose for attending a conference? Question: I am interested in two fields, mathematical biology and cryptography. Is it okay to discuss both of these interests in my statement of purpose? 

I'm currently a junior majoring in mathematics. I have to write a statement of purpose so that I can be able to attend a mathematics conference. During the conference there will be a time where we can submit applications to graduate schools there. I'm not sure if it is for seniors only, but I am writing one just in case. I don't know which schools are going to be there. If I did I would just choose one depending on the degrees offered by each school, but I don't know who is going to be there.
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.