Originally posted by Venturi K10 & K20 won't even trigger the shoe when shutter is set above 1/180s
That's correct. It is by design. K10D & K20D fire flash at above 1/180 sec only in high-speed mode. High-speed flash requires P-TTL.
In general, trying to fire more than one flash unit via sync cord is iffy. The flash units are connected together in a way they are not intended for. Who knows how the flashes behave!
Case in point: I can fire a Metz 45 CT 4 with my K10D and I can fire a Pentax 540. But they the two flashes are connected to the hot shoe (I built an adapter myself), only the Pentax 540 fires. Using a Wein Safe-Sync as an adapter, I can fire either flash by itself. But when I connect both flash units to the Safe-Sync, the Safe-Sync would lock up and does not fire any flash. The Safe-Sync remains locked up for at least 30 minutes afterward. During lock-up, it does not fire even there's only one flash connected to it.
The best bet is to use wireless trigger.