Hello my apologies if I am getting confused but you are trying to solve something by taking a strange detour. Follow these instructions to work in a happy way.
1 create a cube, subdivide it by 3 and apply the modifier. It's just so you have a bit of resolution.
2 in edit mode select all the vertices and move them in the X where the armrest would be left or right where you want.
3 You go into sculpt mode. Pressing N to see the right panel. In that panel at the bottom you see an option that says "Symmetrize" if you display it mark X to make symmetry in X just below that sub-panel you have a button to execute the symmetry if your object disappears is because you have to invert the direction, control z to see the object again and just above the Symmetrize button that you pressed you have a drop-down that says -x to + x because it searches for + x to -x symmetrize again and you will see the two hypothetical parts or armrests.
4 Now to model your two parts at the same time you have to verify that at the top of the toolbar the mirror symbol and xyz appear in three little buttons. It goes without saying that now you have to activate x to mirror.
Two things to keep in mind if you deactivate it, you will stop modeling in the mirror making different wrinkles for the two armrests. Therefore, do not click on the Symmetrize button or you will lose one of the sides replaced by the other and they will be the same again.
The other thing to keep in mind is that if you stop mirroring and make marks on one of the sides, you still have the option to activate the x again but if you model above the parts that are no longer the same, it may be in some area the brush no longer touches the surface if you made an indentation or touch it too much because you made a ridge. In any case, you can do it, just check that you like the result.
As a recommendation, try not to. I would first model everything that I do want to be reflected and then what I don't want to be reflected. Note do not apply uv before modeling. if you press the Simmetryze button this makes a destructive reflex. So that you understand me, the reflection generates a copy of vertices with an index different from the one they had, it did not move them so that they were reflected. I generate them. If your object had uV now your second half in the uv editor are overlapping. Taking advantage of half of your texture map.