I'd go with the two relays approach (I'm guesing one for each side) since the flashers will (for the more recent ones) most likely rely on the relative resistances and having two in paralell could cause them issues, wheras a relay would most likely present very little load to one set of flashers and then be able to drive the other set with no problem.
Older flasher units may very well work in paralell since they will probably be using a bi-metallic strip rather than some kind of resistor/transistor network.
The two flashers won't be in 'parallel' - only one will be in use at a time! One in the supply to the left, and one for the right - dead easy. For a warning light - just connect across the left/right - will flash when either side has volts, earthing through the side with no volts - but connect after the flasher unit on each side.
2 flasher units is the simplest, unless you want to have hazard lights as well, which then needs a single unit and a relay set-up.
If you go the 2 flasher route, you can either have 2 separate dashboard repeater lights, one from each flasher unit, or one by conecting it across each of the flasher unit output terminals. A very unlikely problem is that if BOTH flasher bulbs on one side of the car fail, the warning light will not work!
got it sorted thanks to a pm diagram
adjusted it to fit my car as i think it was a bit diferent.
i had to put 2 little lights on the dash as it just would not work with the bulb on the switch and have tried loads of dif flasher units too
had to use 4x21w in the end one on each side.
its actually quite simply the easiest way is to just splice into the wiring between the switch and semaphores and run that to the L on the flasher unit
on the B run a wire to the front and rear indicators and the little bulb on dash sorted worked a treat.
apart from having faulty flasher units and a bad earth on one of the lights and loads of fun but hey it all works now.