Hello,
Please check the following Blog (very good one)
http://blogs.vmware.com/vsphere/2012/02/vmfs-extents-are-they-bad-or-simply-misunderstood.html
There are lot of considerations that needs to be done while taking the steps. But in general in VMFS5 , it is always good idea to increase the VMFS datastore. Anyways if my memory serves me right then the way it is written in simply concatenation i.e, once one disk is completely full then only the second disk will be utilized so it will not give you much performance boost if you had any in mind.