Untested by me but this is what I have in mind for my car...
Use two 6v flasher relays: One running the left indicators and the other for the right.
The wiring for the semaphores remains untouched.
I plan to connect the flasher relays to the semaphore switch via a double-pole switch (two individual switches operated by one lever if you like). This will enable me to enable/disable the additional flashers. Of course, the switch will be hidden neatly under the bonnet without drilling any holes!
Andrew
195x Deluxe oval Beetle
Binz Doublecab