Eventually, the gap becomes too wide, the arc extinguishes and is reestablished at the bottom. You might as well tap directly off of your utility's substation power bus. This is way to little for a decent Jacob's Ladder and putting them in parallel will not help. Their insulation is not rated for series operation so one or more of the windings will see excessive voltage. There will likely be fireworks - and not where you want them.
Similar comments apply to the use of utility pole or substation transformers.
Microwave oven transformers make excellent Jacob's Ladders and Tesla Coil drivers with a properly chosen series cap in the primary for power factor correction. There are several European web sites that show back to back microwave oven transformer powered ladders. A key problem is that you have to mount the transformers on Lucite or Lexan because the center tap is grounded on the secondary side. They also make nice stable cores for high current low voltage transformers when rewound, I use them to power 25 amp 3.2 volt laser tube filaments all the time with out blowing breakers. Always wanted to build one like this but never had the time, the big resonant column or the really huge neon sign transformer that would have been needed.