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Start with one drive. Create a spanned volume, volume group, and logical volume.
vgcreate STORAGE1 /dev/sdb
lvcreate -l +100%FREE STORAGE1 -n storage1
Add an additional disk (you could have done this in one step, but I want to illustrate adding a disk).
vgextend STORAGE1 /dev/sdc
lvextend -l +100%FREE /dev/STORAGE1/storage1
You can view the status of the physical and logical volumes.