Washington Resident here. I travel I-90 from/to Bellevue to/from North Bend 4/5 days a week since 2000. I have seen many RV double tow rigs pulled over by Washington State Patrol(WSP)and forced to detach the second trailer. I have also seen RV double tow rigs impounded on this stretch of highway.
Lately, one of the WSP trooper's assigned from Issaquah to North Bend almost every morning has a commercial vehicle pulled over for some reason. I would be hesitant to try slipping through this road stretch with a double tow rig from what I have seen.