These transformers are AC Power Supply with 120VAC input and 2kVAC output. Just wire the cores together (the cores are normally one of the High Voltage Terminals) and wire the primaries so they oppose each other. If you don’t get a nice arc from this switch the input leads on one transformer.

This will not arc over that far, but if you start a small arc you can pull it out to about 5″. These transformers will make an excellent supply for a Jacob’s ladder. You will need at least a 20A breaker to run two of these. Watch the ouput of these they are 1000VA a piece. At 500mA at 2kVrms the ouput could easily kill you in an instant. There are several variations of these transformers.

I also have a 4kVrms unit that was taken from an old “radar range” microwave oven. Normally these transformers will also have two leads that come out of them. These leads can be clipped off since they are just low voltage windings that power the filament in the magnetron. Also it is a good idea to use power-factor correction capacitors on these. Use an AC capacitor to do this with a rating of about 30uF.
Source: http://www.geocities.com/capecanaveral/lab/5322/mw-xfrmr.htm