Start with one drive. Create a spanned volume, volume group, and logical volume.

pvcreate /dev/sdb
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).

pvcreate /dev/sdc
vgextend STORAGE1 /dev/sdc
lvextend -l +100%FREE /dev/STORAGE1/storage1

You can view the status of the physical and logical volumes.

pvdisplay
vgdisplay
lvdisplay