I can tell you one fact I DO know. When the military plans a operation,a redunancy of equipment and personell ARE a part of all their planning. This is called contingency planning. They hold equipment and personell in reserve in case they are needed. There is no reason NASA couldn't do the same as a matter of course,and keep another shuttle held in reserve and ready to go within a day (at the MOST) if needed. There is no reason to not have EVA suits and tethers aboard each flight in case the crew needs to examine a part of the shuttle before the decision is made to call in backup,or try to land it. Unlike a lot of the other stuff NASA is involved in,this AIN'T rocket science. It's just basic,fundamental,good planning.
BIG DIFF when you have the manpower and budget to do this ... NASA doesn't ...
Sure would be nice to be able to send up a rescue ship if the one up there has a problem such that it is expected it can't land, but what would having two ships ready simultaneously do to the budget? We would need two of the support items we currently only need one of. Also how would we be able to get the rescue ship to connect with the other one that's in trouble, so the occupants from the one can transfer to the other without going adrift in space? There's nothing to hook.