Most over the road trucks with dual tanks have them connected together and you pull from both at the same time. Helps balance things (100 gallons of diesel is 800lbs) and also stops the numb nuts who used to drive our trucks from running out of fuel when they forget to swap the tanks over.
On most of our dual auxiliary type tanks we ran transfer pumps. Not as good for redundancy but a lot easier to plumb and not as much chance of introducing air into the system.
If you go with a changeover solenoid I would suggest you have dual filter sets and the ability to isolate the filter packs/feeds for redundancy. It is important to make sure the fuel return goes back into the same tank as the fuel is being drawn from. This provides cross contamination protection and also stops you from overfilling one tank and having diesel flow everywhere including all over the vintage bikes in the trailer behind you. Or so they tell me
.
Also make sure both tanks have filters /screens on the inlet/fill point. This stops the bigger lumps getting in and also limits the amount thieves can syphon out of the tanks. One of the hardest problems I had to find was the result of a bit of foil from a fuel additive bottle getting sucked up against the pickup pipe and the once the engine stopped running it would drop off and not get picked up again for some time. Always happened in the most inhospitable places in the middle of the night and took months to find (after chasing all the expensive solutions)
Also checkout something like ProFill out of Australia. They make filter socks for the fill hole that remove moisture and contaminants before they get in the tank. A must for when you are filling from 50 gallon drums etc or dodgy gas stations in other countries.
http://www.profill-australia.com