Ideally, you'll have a changing line and you can attach that to the sail that's being hoisted. Alternatively, you can use the lazy sheet of the hoisted sail and, when the first sail is down, remove the other sheet and attach that as the new sail's lazy sheet.
How do you run that sheet? Extra winches? I only have one outside the cockpit on each side for the jibsheet. All I can envision is having to run the new sheet through the same lead and onto the same winch. I suspect I'm missing something.