Hi,
where does the noise come from?
Is it the EARTH connection? --> use one big common EARTH connection and connect each instrument in a star connection to it.
Is it influenced by the mains cables? --> filters in mains, Filters to a solid and common earth connection, isolating tranformers...
Is it influenced by air via electric or magnetic field? --> shielding
Or is it influenced by the signal connetions from DUT to instruments? --> isolating signal repeater, filters, isolating mains transformers....
Klaus