If they are slabs, then select one and using the pet palette options you can add an area to that slab .
Add the area described by the second slab - either by manually picking its nodes or by magic wanding on it.
Then you have to delete the second slab - it does not merge them.
Or just manually adjust the edges and nodes of one of the slabs and delete the other.
Unfortunately there is no way to merge two slabs.
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10