<Space Island Group wants to build a ring station out of shuttle external tanks. They suggest this can be done by mounting a cargo-carrying ET with re-useable SRBs onto a fully fuelled ET, also with SRBs and additional shuttle engines. The shuttle is not used - this is an unmanned cargo launcher.

Watch their video, you'll see what they mean. It might even be a good idea. The principle challenges in my view are the first attempt to mate two shuttle ETs together and launch them successfully. Next they need to be able to disassemble them and re-configure them in orbit by remote control. Next they need to modify the shuttle to carry 75 passengers (at $500,000 each perhaps?) and also to deliver its ET for attachment to the station.

After the station is done, no more ETs will be needed. So if we can develop a single-stage-to-orbit launcher to carry 75 passengers that would be the goal.