It's a very basic design utilizing an Automotive 12v Flasher, an Automotive 12v Relay, 2 lamps (1156 or 1157 styles) some wire, and a power source.
I did not include a fuse in the schematic, but it should be used. How you decide to switch it is entirely up to you.
What happens here:
The flasher is a bimetal strip. As current passes through it, the 2 different metals heat up and expand at different rates, causing the strip to curl up and break the electrical connection. The lead leaving the flasher is then interrupted and reconnected every second giving it the "Flashing" effect. That lead then runs into the electromagnetic solenoid section of the Relay. The way the field collapses on and off will cause the contacts inside the relay to connect and disconnect accordingly. On an Automotive Relay there are 3 poles to the contactor section. 1 is the input side, the other 2 are output. When one side of the output is hot, the other is not and vice versa. This gives us the opportunity to use the alternating capacity of the relay to make our lights flash opposite of one another. You may note that one of the lamps is attached directly to the flasher as opposed to the opposite pole of the contactor. This is because there must be a load on the flasher in order for it to function (Remember how that strip has to get hot?)
Now, depending on the style of lamp you use 1156 or 1157 are the best choices. If you must use LEDs, you must get a solid state electronic flasher. Reason being the LEDs do not have enough of a current draw to overheat that "Strip" inside the flasher.
The 1156 is a single filament bulb. The 1157 bulb is a double filament. The wiring difference is minimal between the two schematics. If you choose to use 1157's you can wire it so that while one light is flashing bright, the other one is illuminated with the dimmer filament and alternates between the two.
This is the simplest way I can come up with to get your lights to flash alternatively.
I hope these links work. When it comes to analog technology, I'm tops.... but this modern stuff doesn't always cooperate.
http://i779.photobucket.com/albums/yy74 ... quired.jpg
http://i779.photobucket.com/albums/yy74 ... ematic.jpg