I have a pair of Yongnuo RF-603 radio flash transceivers, two SB-600s & D200. I'm planning to build a large rectangular softbox that'll use both speedlights for extra power. Is there a synch cord splitter or other DIY method of triggering two flashes from the one hotshoe or synch port on the receiving trigger? I realize I can use the D200 to master the two speedlights via CLS, but this can be dicey if I'm outdoors and lose line-of-sight, or use a long lens at long distance, or embed the lights within the softbox.
Another option to consider is getting one pair of radio triggers and then using an optical device like the Wein Peanut to make the second flash into a slave, if it doesn't already have that capability. I've used this kind of setup to fire extra flashes and stretch my existing wireless trigger capability.
Note that if you go with the wired trigger bridged off a single wireless device that the trigger voltage for the two flashes should be the same to avoid any problems with the flash units. Since you mention that you're using two flashes of the same type you're covered.
If you have the PC-Sync cable connectors (either directly to the camera and flashes or indirectly through a hot-shoe converter), and have minimal soldering skills, you can cut a PC-Sync cable and solder standard audio 3.5mm phone jacks so you can disconnect it and do all sorts of nice things. For example, use a standard phones extension cables to extend the range of the flashes or use an audio splitter to split the PC-Sync to a Y form - to connect a camera to two flashes.
As for the radio triggers, it seems like all the cheapo ones have 4 channels or more to choose from. I will not be surprised if receivers of the same model share the same frequencies so one transmitter can trigger two or more receivers.
I am assuming the yongnuo trigger is TTL judging from the hotshoe pics, sadly the pdf on their site isn't loading for me. If you wanted to go the wired route and connect to two flashes together I can recommend the latest Yongnuo FC-682/M TTL cord. Unlike it's older model, the new one has two TTL hotshoe mounts so that once mounted on the wireless trigger you can mount one flash onto that, and then another at the end of the cord. Obviously doesn't give you as much reach as having another wireless receiver but if you're mainly going to use it for a softbox this shouldn't be a problem