Rule 1You can only use 2nd curtain flash in 'local' mode - i.e. with Wireless func. disabled. This limits various things, like camera control of flash power by groups, but is by no means a show stopper, and certainly doesn't stop the use of off camera flash, or multiple flashes.
I'm only interested in manual use of flash.So I haven't tried out anything related to auto flash things.
Things that work:
simplest use: built in flash plus extrasUsing any flash fitted with an optical slave trigger, on camera flash can be turned down quite a bit (32nd works where other flashes to be triggered are within a few feet annd not too far off axis). Suck it and see.
I've done this with some studio flashes and using an old Soligor flash synchro unit which triggered an old Sunpack flash. (Can't even find an internet ref to the slave device, but it works well)
If the on camera flash is a problem, I switch to using Pixel Kings to do the first triggering.
MORE POWER: Canon flash in hotshoeI can plug a speedlite into the camera hotshoe to get a bit more power or point the flash up for some bounce, and trigger further flashes optically as with the built-in flash.
Using a ttl flash extender cableI've not done this myself, but this would be the logical next step if the length limitations aren't an issue (and a lot cheaper than wireless remotess..)
Using wireless (radio) remote triggers - Pixel KingsThis gives slightly more control over at least a couple of flash units.
I have 2 Pixel King receivers working with 2 Canon 270EX IIs. The flash power of both can be set through the in-camera menu, but of course this sets both to the same power (but is still a lot better than nowt). Further flashes can of course be triggered from these if they use / are fitted with optical slaves.