For one if you going to macro it the second part of Soul Swap is actually Soul Swap: Exhale. What I do is I have a focus modifier turned on in my interface settings so when I hold Shift my spells hit my focus. I keep my secondary target focused then hit Soul Swap and then hold shift and hit it again.
To eliminate an extra button that I need to have on my main quickbars...I macro'd soul swap into one of my spells.
/cast [mod] Soul Swap; Haunt
I'm at work..but I believe that is the correct format. Anyways...all i do is press Alt + Haunt, and then click the other target and press Alt+Haunt again. I have also set SS on a miscellaneous quickbar so i can see if it is available to use. I realize there are addons that can show you your cooldowns and whatnot....but this works for me.