Code:
/target focus
/targetlasttarget
/focus
/targetlasttarget
That should swap Focus and Target
F=Focus T=Target L=Last target
F T L
1 2 x --initial x is nothing
1 1 2 --first step to put target in last target position and focus target in the target and focus position
1 2 1 --focus and target are now back to original, with focus in the last target position
2 2 1 --original target is now focus with original focus still in last target spot
2 1 2 --target and focus have now been swapped

The above should happen instantly. It has its quirks but it works, just don't spam it, and make sure you start with a focus.

I hope that helps.