Just got back from a great supported cycling trip to Death Valley.
There are at least a couple outfits that do these tours, we went with REI Adventures. Normally the bikes are provided by the tour company but you can bring your own. Of course when they say that they are NOT thinking about you bringing a tandem! But after some back and forth they thought we could do it and it worked out to not really be an issue at all. We just used a back position on their roof rack and let the tandem extend to the rear of the platform.
Day 1: Start out with the Big Drive from Las Vegas to the far northern end of Death Valley. Begin the fun with a tour of Scotty's Castle which is famous more for the story than the house actually. Have a picnic lunch and then get on the bikes for a 45 mile ride down into the valley and to Stovepipe Wells. This ride was a tandem dream with 3000 feet of elevation drop followed by rollers. This picture was the end of the day/ride at Stovepipe Wells:
Day 2: Ferry the bikes from Stovepipe Wells to Furnace Creek. Ride from Furnace Creek down to the Artist Pallette loop, ride up and around the loop, and then continue down to Badwater. Have a late lunch and ferry the bikes back to Stovepipe Wells with stops at Devil's Golf Course and the Sand Dunes. Lots more tandem friendly rollers on this ride with a couple of butt kicking 7+% grade climbs on the Artist Pallette loop. We broke down and had the bike ferried to the top of the first big 2.5 mile climb and I pushed to the top of the second big climb. About 35 miles this day.
Day 3: Ferry the bikes from Stovepipe Wells to the base of the road to Dante's View. It is only 13 miles from the junction at 2000' up to the top at 5500'. The first 7 miles runs 3-5% grade which is quite doable. The next 4 miles runs 7-8% grade which was more than we were up to on the third of biking. And the last couple miles goes up to 10% with the last half mile at 12+%. Definitely not a tandem ride going up. Coming down is a good brake test. Finish up with a lunch and drive back to Las Vegas.
We decided that over 30 MPH we did not have to pedal, we had lots of time without pedalling
If you want to see the sights of Death Valley... and like to cycle... this is a great trip. So far as I know REI is a cheaper option and the only one to include Scotty's Castle in the itinerary. They stay at the more basic Stovepipe Wells which is nothing fancy but quite functional and has a bar/store/restaurant to enjoy downtime. Backroads stays at the Furnace Creek Inn for more $$$. From what I experienced and read both will allow you to scale the ride to whatever level you want.
There is an art to timing this trip: the first two days we were there the daily high temperature was about 80 which was perfect. Our last day it dropped at least 10 degrees plus wind chill. Add on the drop at the higher elevation of Dante's View and it was downright cold coming down. Everyone knows Death Valley gets hot in the summer but it also gets cold in the winter. Hit it in between and enjoy.
Dante's View (sorry no OBG tandem, we elected to skip going down 12+% grade):