This comes up from time to time...
The 320, 306 and 280 were the first three to get the two Stripe Paint Scheme. 284 soon after that. There is a well documented Rocky Mountian Club sponsered trip in May 30th 1952 where just the first three coaches have the Two-Stripe Paint and poor 284 was looking a little ragged in old green paint. Soon after this trip the Coach 280 was donated to City of Boulder and 284 was painted with the Two-Stripe. I don't know if there was any overlap with both 280 and 284 being two stripers at the same time.
After the May 52' Trip, Combine 212 and baggage car 126 also got the Two-Stripe paint and these five cars (126, 212, 284, 306, 320) remained like this until 1955 when all the remaining passenger equipment recieved the Silverton Yellow paint. All the equipment got One-Stripe in 1955.
I love the Two-Stripe because of the silver platforms and trucks and undercarrage. Probably a nightmare to keep clean though.
Hope this helps
Kevin Cook