I recall a comment from the tunning guide about crew holding main sheet and traveller and skipper with the jib while upwind and double trap. Is that so? Any comments?

???Why would they switch who-does-what? Who's got the tiller?

Just quoting+translating the "Sailing guide" by Roth & Monfret a pdf also referenced as tunning guide:

"Skipper: ... On a broad reach I have the tiller (of course) and the jib sheet, that allows me to trap very aft. The jib sheet passes over the main sheet, managed by the crew"

"Crew: upwind I have the mainsheet (good gloves required)... ... on a broad reach I have the mainsheet and traveller"

Well, that is my translating and not sure if i got it right.

I see. I suppose it sort of makes sense but it seems to imply that the skipper is using the jibsheet as a support (otherwise why couldn't he 'trap very aft' holding the mainsheet instead of the jibsheet) rather than a control.
I think the skipper should have the primary controls of the boat (main & tiller) to be better able to react quickly even if it means he can't trap quite as far aft (although I don't really see why not). If there is still a need for more weight aft, the crew should also move aft as much as possible.

The only reason I could see for doing this, is to provide the skipper something to hold onto when the windward hull dropps suddenly on the trapezze, thus preventing the skipper from flying off the back of the boat, and the time it takes to recover. The mainsheet offers no help in this situation because it just swivels around as you go flying off the stern. While at the same time with the crew holding the mainsheet, if the lee bow burries, the crew has support from going around the forestay (However this sounds dangerous here, if your on a broad reach or going to windward, and burry a hull, you would want to let OUT the mainsheet, not have the crew pull it in to prevent from flying forward.)

I would think the suggestion of having a line tied to the middle of the tramp for the skipper and crew to get support from while out on the trap a much better idea. As a the skipper you have much more control over the boat, being able to adjust the mainsheet to any steering you might have to do.

Just my opinnion,

Brent

When I crew in a race, I most often run the main and jib, while the skipper runs the tiller and traveler.
I trap out first, and if there are double-trap conditions we usually overlap our legs to keep the weight together. This has the benefit of being able to brace against each other, or hang onto each other, if absolutely needed. I really prefer crewing in a race, I'm much better at it.

If I skipper in a race, it usually means that nobody needed crew, and the person I have crewing for is on the boat because I told them we were just going out for a sail and they have no idea what's going on. I give them brief instruction on running the jib and trapezing and pretty much just tell them what to do and where to be during the race. I run the tiller, main, and traveler (my mainsheet is also my traveler line). I still let the crew trapeze first, but on my boat there are foot-loops (garden hose and old halyard version) to help keep you in place. I try not to ever let them know we're actually racing, or they would start asking why I'm putting around behind everyone else. I really don't like playing skipper in a race, since I don't have much clue about where I'm going half the time.

I helm and always have the main upwind. It's not a Tornado you can always one arm it. My team-mate hold the traveler. She holds it so I can grab it quickly if I need to ease before a puff. I think this works well for us.