I've seen this clip of Alan Goldberg chi sauing a few times in the past, but this is the first time I've been able to pick out something I could use. It's pretty interesting to watch all the way through, but the "combination" I'm going to use starts at 36 seconds:

Let's pretend you're Alan Goldberg, doing the technique exactly as he shows it. Here is the run down:

1. you start with one hand inside and one hand outside and just roll
2. as your right tan sau changes to the outside on the way up, punch forward with it (instead of changing to a fuk sau, as you normally would).
3. this punch is not an attempt to hit your partner; it's simply to provoke a bit of resistance from them.
4. as they push back against your punch, laap their resisting hand with your punching hand.
5. bring your left hand over to laap their arm (the same one). The good thing here is that your left arm is covering both of theirs. One hand controls two and you have a spare hand.
6. draw your right hand back only enough to punch them for real this time.

If you look at the video again, you can see how smooth Goldberg is in his actions. He's obviously done this move a few times. It's a fairly simple one to introduce to your own chi sau, so I'll be using this in my next class.

You could do your initial right hand punch at any time as long as that hand is on the outside. The reason Goldberg does it immediately after changing from in to out is to "mask" what he's actually doing. His partner is used to the changing action from in to out and so thinks that's all that's going on.

As with most chi sau techniques, this one isn't a killer technique that you can use to "finish it". If anything, it's a tool to get you used to a concept that you can transfer to other situations. What's the concept? Push to provoke some resistance from your partner and then take advantage of their energy. The technique won't work unless your partner uses their strength to push against you. If your partner's Wing Chun is any good, they probably won't be pushing against you, so you have to create a situation where they are. That is, push them first.