Which optical slave with ETTL?
There are some hot-shoe flash optical triggers available that can be programed to ignore the TTL pre-flashes but I have no idea how well they work. Here is one example.
I see little reason to use an off-camera flash in TTL mode unless the subject to flash distance is constantly changing. For me a simple budget priced RF trigger works fine. I recommend the Yongnuo RF-603s.
I would say the time is past for the 420EX since it doesn't have a manual power control mode. If you want to use it off camera the only way I see it to use both your flashes off-camera in E-TTL mode.
Living and loving it in Bangkok, Thailand. Canon 7D - See the gear list for the rest.