Have to run the numbers, taking into account all the costs. Two vehicles to insure and register. Might be twice the upkeep too, if things don't wear out they can age out. And you might find they both decide to be in and out of the shop (yours or elsewhere) on a regular basis, due to the random nature of things breaking.
But then there is a convenience factor of having a second set of wheels for trips to the parts store, or while waiting for parts in the mail.
Gas a bit cheaper when I got my beater 4 years ago, but I calculated it to be break even. 19mpg truck vs 30mpg car, plus the reg&ins on second vehicle (ignoring depreciation and repairs). Break even or close enough. But having a spare means I can repair vehicles when I want to, or to have one sit at the shop if need be. It's also nice having a car that I don't care about (much), can park anywhere and not worry about dings.