Like
turbo said; you doubled up on the wrong capacitor. The big-ass cap is charged to 300~330V by the circuit board. That voltage will only jump a gap of about 0.2mm (~1/100"). There is a lot of energy in the cap but the voltage just isn't high enough to spark. (Two photocaps would contain about 12 joules of energy when fully charged.)
The high voltage (about 10KV) needed to trigger the phototube is created by dumping a second, much smaller cap (see photo), through the HV trigger transformer. The output of that transformer is carried by a wire to either the metal reflector around the flashtube or to a wire wrapped around the flashtube. That wire is the only source of high enough voltage to jump a gap. You can try to use that wire as the spark source. The other side of the gap would be wired to the circuit boards ground contact. Though the spark is up around 10KV the current is extremely low because the cap that supplies the power is only something like 0.02 MFD, total power when charged to 320V is 0.5millijoules (just barely enough to ignite propane+air).
So, it is that small cap that you might try doubling up using the cap from another board.
Or, dump the big-ass photocap through a car ignition coil. You might think you could dump the big-ass cap through the trigger transformer but I suspect you would destroy the transformer with that much power going through it.
Or, try a
trigatron, which uses the two leads from the big-ass cap plus the HV lead in a three electrode spark gap. That'll give a huge spark, but is a PITA to keep working becuase the alignment is tricky and the electrodes get burned up.
Or, like others sugested, just use a BBQ piezo or buy a stungun.
