IF, the logical volumes are on different volume groups and you want to merge them, use the
First, deactivate the volume group that will be merged into another:
[root@host ~]# vgchange -a n vg_to_be_merged
After that, test if there is no inconsistence that will prevent you from merging:
[root@host ~]# vgmerge -t -v dest_vg vg_to_be_merged
Now, do the merge for real:
[root@host ~]# vgmerge -v dest_vg vg_to_be_merged
Checking for volume group "dest_vg"
Checking for volume group "vg_to_be_merged"
Archiving volume group "vg_to_be_merged" metadata (seqno 2).
Archiving volume group "dest_vg" metadata (seqno 13).
Writing out updated volume group
Creating volume group backup "/etc/lvm/backup/dest_vg" (seqno 14).
Volume group "vg_to_be_merged" successfully merged into "dest_vg"
However, merging LVM volumes will have no magic trick, since they have filesystems on top of them, and a lot of thinkering with data moving and resizing will be needed.