Step one when using multiple transformers, even if you don't initially need to, is always to "phase" them.
The process is simple, and has been discussed here hundreds of times. You can do a search for "phasing transformers." The process ensures that the high peaks and low peaks of alternating current from multiple sources coincide, and do not fight each other.
In your case, one side of the switch motor 18Vac output needs to be connected to the outside rail (RETURN) supply of the traction transformer. (This transformer is the one you will use to make the trains go.) The other side of the 18Vac gets connected to the switch motor pins (the HOT side).
Once the transformers are in phase, you will never get a short or heavy spark, or any other electrical problem related to the fact that you are using more than one transformer.
If you are using a traditional transformer on a relatively small layout, you can also consider using one output from a multiple-output transformer, such as the old ZW type. In that way, you needn't worry about phasing, because all the outputs are in phase by design.
Be sure to equip the switches with lamps that can handle 18 Volts, or you will get overheated and melted lanterns. if you are using old tubular switches, you can equip them with 24-volt lamps and not worry about the heat. Type 1437 draws 0.06 Amp, and is rated for 24-28 Volts.
