I'm sure I watched a documentary that said it basically wasn't feasible to launch the other shuttle. All checks and preparations would have to be done in absolute record time, with no mistakes and under timelines never attempted before. But even if they tried, you have the obvious question of - we know the core issue isn't solved and we're about to launch the second shuttle with the exact same design into orbit, if it suffers the same problem then what? But afaik the second one while important wasn't as much of a blocker as the first one. It just wasn't possible in time - it's not like the first shuttle could stay in orbit indefinitely too.