I'm interested in this as well. I believe you need to sign up for the Romance Rewards and schedule it once you arrive. It appears to be a first-come first-serve basis, with limited room (I read somewhere a maximum of 10 people). It seems to imply you can only do it once, but someone reported doing it almost every day because they preferred one of the other beaches.
I'm interested in learning about the mechanics of it. Is transportation provided? If not, will they help you schedule a drop off/pick up with someone? Are you excluded from activities/food? And just generally interested in how well it works in people's personal experiences.
We traded from Tower to Souci in 2010. There was probably a dozen of us in the Couples shuttle, at no cost. They bussed us over maybe 10am and picked us up maybe 4pm.
I'm working on trading from Souci to Tower in a couple weeks so I will post if I learn more.