I would love to see 2 trains per day, in each direction, on a long distance Amtrak route. If double frequency was to come about, there would obviously be concerns over cost, farebox recovery, and potential ridership. You would have to juggle between desired times of departure and arrival at various towns and cities. Also, times for passing through scenic areas along the way.
I would pick the Coast Starlight as a candidate for double frequency. I think it could draw a good ridership with the many cities, towns, and communities along its route. I also like both a morning and evening train for each direction. However, that could be difficult.
For example, if a 2nd CS #11 southbound left Seattle in the early to mid evening, it would be very late getting into Portland and Eugene. If CS #14 northbound arrived into the Bay Area around 9-10 AM (starting out at 9-10 PM from LA) , it would be great for hopping the northbound train from the Bay Area and for arrivals at northern Calif. stops, north of Sacramento, at reasonable hours. But, chances are it would be dark from Klamath Falls to Eugene, one of the most scenic stretches along the entire Starlight route. Also, a very early morning arrival into Portland.