The funny thing is, the actual industry is going in the opposite direction, because it turns out that a single big tunnel is in practice cheaper than a bunch of small ones. Conventional deep-bore metro lines consisted of two tunnels about 3.5 meters in diameter, but if you look at _current_ metro projects, the Dublin metro is using a single 10 meter tunnel, the newer Barcelona lines are using a singe 12 meter tunnel...

The expensive bit of building metros isn't actually boring the tunnel, generally, not anymore. It's everything around that; securing a route, the disruption while it's going on, etc etc. So the last thing you want is to have to do it twice.