How to attenuate multiple signals

I want to automate the testing of transformers and be able to take all voltage measurements. For transformers that we have, this includes up to 22 inputs and outputs combined. I have a Digital multimeter with 1 input and 1 output. It will be hooked up to a multiplexer and then the multiplexer will be hooked up to the transformer under test. My problem right now is that the transformers go up to 600 Volts, and my DMM and multiplexer will only allow up to 300 Volts. Right now, I have the inputs and outputs of the transformer under test hooked up to one potential transformer, and I have relays for each input and output. Is there a more efficient and easiery way to do this? Any help would be great.

Thanks

Please would you post a circuit diagram of your present setup to make the situation clearer ?