Quoted verbatim from https://www.oregon.gov/odot/Safety/D...ngATrailer.pdf
Number of Vehicles in combination without special permits:
A vehicle may tow only one vehicle or trailer in Oregon (a two-vehicle combination ORS 818.110). Some states allow towing more than one trailer or vehicle by non-commercial vehicles. Permits are not allowed in Oregon for towing more than one trailer or vehicle except for commercially titled and registered vehicles.
In the PDF the paragraph is on page 18, however it is actually page 16 of the original document.