Double eyepiece holder for two person observing is described in the attached catalog. It was available in a Model A for .965″ eyepieces and a Model B for 1.25″ eyepieces.
This is one of the early advertisements for the Duetron (October, 1959):
Photos courtesy of Xavier Debeerst
This second set of photos is of a 1950’s unbranded version of the Duetron Model B. This is the larger Duetron for two 1.25” eyepieces. It has a chrome finish and includes a blue filter for the second observer.
Some additional photos of a Polarex Duetron Courtesy of Stephen Brennan: