Except that in tunnel boring the circumference is not that relevant, it's the surface area of the cross section of your tunnel. In the first instance that's 113.1 square feet, in the second it's 452.4. Every time the radius doubles, the surface area quadruples.
The perfect tunnel transports people horizontally in a very narrow tube.
Your forgetting, I'm talking about either two lanes of traffic in one tunnel or one lane of traffic in two tunnels.
First order analysis says there is no saving by using two tunnels instead of one. When you consider 4 one lane tunnels vs one tunnel with four lanes of tunnels, the four tunnel solution is worse.
Single lane, 12 feet wide, 12 feet tall. Requires a tube 17 feet in diameter. Or 54.3 feet circumference. Two of those is then 108.6 feet.
A tube that can carry two lanes, 12X2 = 24 feet. Diameter is 26.8 feet or 84.3 ft circumference.