From the 5.5 Admin Guide
http://docs.vmd.citrix.com/XenServer/5.5.0/1.0/en_gb/reference.html#id2554601
3.2.6. Storage Multipathing
Dynamic multipathing support is available for Fibre Channel and iSCSI storage backends. By default, it uses round-robin mode load balancing, so both routes have active traffic on them during normal operation. You can enable multipathing in XenCenter or on the xe CLI.

 That document is still valid. Just keep in mind that XenServer uses all routes available to SAN. The controller on the SAN if configurable can decide which port(destination path) writes to the LUN. For example let’s say the customer has 4 paths. Traffic goes over all 4 paths to SAN array. Controller on SAN array get the traffic and then decides which controller port will be used to write to LUN. Most SAN arrays (that I know of) only support one controller port at a time writing to a LUN. Some may round robin traffic or some may be able to assign a specific one. In the end though the SAN manufacturer will have some documentation on how to configure both from the SAN array and if any configuration other than default needs to be done on a Linux OS (preferable RHEL 5.x).

I attached an example of the type of document you’re looking for from the SAN manufacturer along with this link
http://support.novell.com/techcenter/sdb/en/2005/04/sles_multipathing.html