I've never used serato by i guess it's pretty much the same as in traktor, so let me try to help.
DJ number 1 is playing with serato timecode and one audio interface box between cdjs and mixer. DJ number 2 is ready to take over and plugs two usb cables directly from the laptop qith traktor to the cdjs.
DJ 1 finishes playing from CDJ 1 and mixes to CDJ 2. DJ1 puts the serato audio interface in by-pass mode (in Traktor it's called direct thru, so I guess serato has the same option). DJ 2 pushes the link button on CDJ 1 and connects it to the deck 1 of Traktor. When the track on CDJ 2 is finishing, DJ 2 mixes in the track on CDJ 1. DJ 2 can now also push link button on CDJ 2 and connects it to the deck 2 of Traktor.
DJ 1 is now free, can remove the timecode CDs from the CDJs and also start removing the audio interface: first unplugs the channel connected to CDJ2 that is not playing and reconnects the CDJ directly to mixer. When DJ 2 mixes into CDJ2, the DJ 1 can finally remove the interface and connect the CDJ 1 directly to the mixer.
If the booth is very small and cramped and there's no space to have two laptops open at the same time, the DJ 2 might instead use a USB stick and play with that until the DJ 1 has taken away the hardware and then finally install the traktor laptop, as above the only thing to coordinate is that DJ1 unplugs the channel of the audio interface where the DJ 2 is not playing.