Hope this is readable,found these in a file. Actually,I think it can be done with SPDT relays. click on photo to enlarge
Instead of 3 throttles a voltage dropper can be used
A bit of warning about what you are doing now. If your 2 throttles set at different voltages are somehow hooked together, you can damage your transformer and it is not breaker protected. Note the the relay contacts keep the throttles separated electrically from each other. I do recommend the wire wound resistors mentioned either. In a track short circuit,they can burn up,also not breaker protected sometimes. Also different locos will react differently at the same setting.
Dale H