conflicting sizes compression fittings dishwasher/ washing machine adaptor

Hi
I want to attach a dual spigot adaptor to accommodate two appliances I have a fitting but the white plastic compression nut on the fitting is too small to screw on to the trap assembly -the trap assembly nut size appears to be 55mm diameter and so is too big.
Is their a particular make that has this bigger size ? I can't seem to fin one on line that matches the existing trap
Thanks