Unless you are doing two completely separate transactions-and by that I mean, close on selling your house AND THEN start looking for another house; I think it would be better to have a single agent. If you are like most people and are going to start looking before your house sale CLOSES, then one agent would be best as they know where everything in the process is. If you have two agents, your buy agent is going to want to know where your sale stands and you will have to contact your sale agent and pass information on. Just make it easier and use the same agent, especially if it is a friend.
Also, unless it's different in your area, you only pay the agent when you sell and the seller pays both agents. So if she will lower her cost on the sale because she will make $$ shortly thereafter from your buy, that saves you money over going with separate agents . You should still pay the full amount on your sale to the buyers agent though.