Hello and welcome to the forums.
When you initiate the SVMotion, using the Migrate Virtual Machine wizard, choose the Advanced >> option on the "Select Datastore" screen. This will allow you to move the VMDKS individually. You can't split the datastores using the "Edit Settings" options.
Good Luck!
Brian Atkinson | vExpert | VMTN Moderator | Author of "VCP5-DCV VMware Certified Professional-Data Center Virtualization on vSphere 5.5 Study Guide: VCP-550" | @vmroyale | http://vmroyale.com