You can solve this by using Relative servo from the servo section of the add items in ARC.

or set up animation frames for the action.

yes thru,but a checkbox is so easy in use.

In theory mirroring with a simple check box may look good however in practice, the left arm at position 60 may not be the same as the right arm at position 120. You may need to adjust them individually depending on the actual position of the servo and any play in the fixings and mechanics.

In practice it may be left arm at 60 and right arm at 122, in which case a simple mirroring button would not provide satisfactory results.

in ghoreograph they work perfect. ok so its not yet for ARC then.

thank you