Once you all are paired up I suggest PMing and figuring out what site you will use. In an old RP I was in collabing would be used often and had these rules;

 1. Collabs must be at least 4 paragraphs with each player having the same number as posts as the other but the length should not exceed 8 paragraphs.
 2. the post paragraphs must have decent length but not to much. (Usually my partner and I would right about 3 lines and then pass it onto the next)
 3. Balance dodging and receiving hits to the point where that person is a realistic opponent. 
 4. Each team will have 3 posts to complete their battle, inbetween these posts other members of the RP can post brief reactions or words of encouragement to the players who will react as best as they can. 

 Thats just how I was taught to do collabs.