Doesn't fire both at once, and you could look at the slipring as you suggest. But easier just to know that the lower lobe of the camring fires the rear pick-up on the magneto (gearbox side). So when the points are just opening at about 10 or 11 o'clock (looking from the primary drive side as it's the only side you can see from!), the heel on the moving point is opposite, about 4 or 5 o'clock, and the rear pick-up is firing. So, if (say) the rh cylinder is on compression, piston set before tdc as desired, and points just opening with heel on lower ramp, you're most of the way there.
With the spark plugs out you can see if you're on compression or exhaust stroke, as open valves are easily visible with a small torch.
If you get the sides wrong but everything else right, swapping the plug leads sorts it out. There's no law that says one pick-up should always go to one cylinder. I go 'rear to the right' but that's just preference. Cheers, Bill