Sorting entities in a view by a field in a 'sibling' entity
(1) you cannot do this simply by editing the view, but (2) you can do this with a custom module. I expand on (1) and (2) below.

(1) holds because there is no single ;-terminated SQL query which will do this. The reason is that when you have 2 profile2 profiles the data you're fetching is spread across 4 tables structured like this. As you can see from that, the 2 profile2 tables reside in separate tables, and you can't tell that they belong to the same user except by checking a 3rd table. SQL queries simply can't manage this.

(2) is possible because a custom module lets you run multiple lines of PHP, drawing on additional query to fetch the field you want to sort by. You want to do this in hook_views_pre_render. Here's a module I created to do something similar on an existing site I had: http://www.pastebucket.com/40183