Off camera flashes
Generally speaking, set the Yong. flash to slave mode, and make sure the Metz flash is NOT in an FP mode(if it has that option) on the E-620. You may have to experiment to find the best sync speed for the two flashes.
To use an example, using an improvised setup like this I had to set a camera to 1/60 in order for all the flashes and the camera to sync up. Radio triggers with built in hot shoes triggers tend to be faster to sync, but you will still probably be limited to 1/160 or 1/180.