PS That being said, IMHO even a solo owner-operator can justify an extra truck if only to be a back-up unit for the inevitable breakdowns of his main unit. For example, as an owner-operator when I upgraded to my second TM I kept my older van/unit as a backup. (We all know the almost zero value of a used TM!) Then when I hired my first assistant the machine was available for him to run in emergencies and on big jobs the two of us used both machines. When I sold the company we had three almost TM's in almost constant use and the best of our older units stayed sitting by the shop for those very profitable "same day jobs".
I agree with Steve when you only have one machine you're putting a lot of trust in something that has a potential for break downs. When it's down you're out of business, with a backup even if it's not the newest technology you can keep running.
It seems like when you get that second machine you get more work. Maybe it's just work that passed you by and you didn't notice before? You can also bid on larger jobs with more confidence, and process them faster when you get them.
If you're consistently booked up a week in advance, you're losing work to someone. Maybe it's time for a second truck, remember the second unit doesn't have to be a big investment. There is lots of good used equipment out there, unless you want to do as Steve mentioned upgrade to a new unit and move your existing unit to backup.