In case anyone just needs for space, probably this helps: Extend SailfishOS rootfs with a hidden, unused partition.-Oskar Roesler's Blog
But to the actual question. You don’t need to merge the 2 logical volumes, that’s not how LVM works. If you want to use the SD-Card space for root & home part, you can add the SD-card as a physcial volume to the volume group of the LVM, then assign the new space in the volume group to logical volumes. I don’t know at which layer Jolla deployed LUKS so watch out to not destroy it. To learn more about LVM, this will probably help you the most: LVM - ArchWiki
Always back up before doing anything!