Phasing transformers

search the forum, thats how i learned to do mine,  so what I learned is hook the U  posts together. then get a multimeter and set the voltage pretty close between the two A posts or whatever on the two transformers. then take a jumper wire and touch the A posts together. I say A post because on my KW the A&B are controlled by the handles.  it should make a very little spark, or none at all. if it does....... and you will know if it does..... then take the plug in the wall and flip it around the other way.  theres also youtube videos if you search there......  I phased my KW and 2 1033's this way. 

Are your exiting ZW's phased? (They need to be, just like the new KW)

For phasing, I tie the commons together, than take a variable channel half way up on each transformer and get a voltage reading across the variable channel outputs.. If the voltage reading is consistent essentially 0, you are in phase. Otherwise, the transformers are out of phase. There are variations of this that use a light or a spark and such...

If you find your transformers are out of phase, take one and literally unplug it form the wall outlet and rotate the plug 180 degrees.

Mike Mike Reagan (at Lionel at the time) put a video:

Only flip 1 transformer's plug.    Once they are in phase, I would mark the plugs so if you ever have to unplug them you can easily plug them back in correctly.

If you are already using a multimeter to set the transformer's outputs as mentioned above, you can then use it to test from the "A" of one transformer to the "A" of the other.  The voltage will be either near zero (in phase) or double of what you set (out of phase).

I did this many years ago.  Might be useful.  Phasing Transformers.

I phase them as above in a power strip and then leave the transformer plugs permanent. I disconnect power strip from wall to kill all power.

Do yourself a favor and MARK THE PLUG anyhow as Marty Suggested. Trust me the time will come when you will be happy you did.