Some of them only operate as Sole Seller. They won't agree to anyone else selling at the same time.
You'd have to ask them. To be honest they'll both be putting a sign up, they'll both be putting an ad on Daft.
The difference really is, have they a good base of buyers?
Do they proactively come back to people they know are looking to buy?
Do they follow up people who have viewed houses to see if they are interested?
From my experience as a househunter, it is the bigger EAs only who still keep contact with me to see am I still looking.
Also they are the only ones who follow up when I've viewed a house (though the market is very quiet at the moment, I haven't viewed any houses since the spring).
I just think the bigger EAs have the office staff and the bandwidth to chase buyers and to keep in touch with the reality of the market. The smaller guys never come back to me to tell me about houses I might be interested in.