Menu "View" - "Swap selected" is available when more then one person is selected.

It switches the positions of the selected frames relative to the vertical axis. This is a useful tool when manually arranging a tree after importing it from a GEDCOM file. Here are a couple of examples:

1) Example of spouse positions after a GEDCOM file import:

After a GEDCOM file import

After selecting the couple on the left and swapping their places, this piece of the tree looks a bit better:

After swaping

2) Both husbands of Susan Shield are happen to be on one side of her:

Before swaping

Select her and the nearest spouse and swap - now the lines to their descendants are better seen. Swapping the other pair as well will also help tidy up this section of the tree:

After one more swaping

The hot key for this menu item is Ctrl+W.

See also:
Menu "Edit" - "Undo move"