Do not run cron on both servers.
Currently in my work, I have a similar situation, with Varnish (cache and load balance) + 4 EC2 servers (Nginx) + RDS + Redis (for session and cache). One Ec2 running just to adminhtml panel (admin with a subdomain e.g. admin.mystore.com.br/admin) and three EC2 to store frontend (www.mystore.com.br). We work on all the admin server and update the remaining server with rsync 2 in 2 minutes.
All our crons runs on the administration server, so in this case the sitemap.xml always created at admin server and replicated to other.
This approach makes it more scalable solution than pure rsync (without a master server - "server admin" in my case) or scp because the number of web server that you own can raise all you want.
Of course, in a cluster solution, there is many "variables" to consider, for example, remember to synchronize the media folder because if you do not have a centralized admin will have problems because when uploading an image in admin panel, you'll be saving the image only in one of the servers, so 404 for many users when they hit other server.
Finally, if you want just a solution to your current cluster, use scp like Sander described in first answer.