Can they actually dock two Orbiters to ISS?

The easy answer is, "don't". Undock the 'broken' orbiter and move it away from the station before the second one docks. If they aren't planning on repairing it, it will probably be deorbited before long anyway.

The presence of a "safe haven" to save the lives of the astronauts is priceless, of course. That said, loss of another orbiter even if the crew is saved would be a staggering blow to the space program and ISS completion.

Therefore, I sure hope NASA doesn't plan on sending up orbiters in reliance on the ISS safe haven and that means being very very careful before each launch and that means a 2010 deadline for ISS completion is quite likely unrealistic.

And that means, the STS budget will extend beyond 2011 and delay the transfer of shuttle savings to Project Constellation/CEV. Will that delay return to the Moon?

Flying shuttle C by 2008 reduces the need for orbiter flights perhaps by 75%. Send an orbiter to ISS (with an ISS component payload) and while its on orbit, send a shuttle C with perhaps 3 ISS component payloads. The orbiter meets up with shuttle C and offloads the ISS components.

Since on orbit assembly appears to be the basis of the EELV Project Constellation, why not practice with shuttle C and ISS?

Astronautix says shuttle C can lift three times the orbiter. If true, then 6 orbiter missions plus 6 shuttle C essentially finishes ISS.

Shuttle C's payload bay, however, is by design the same size as the Orbiter's. This was done to provide payload commonality, which means the ISS components built for Shuttle launch *could* be moved over to (an as-yet hypothetical) Shuttle C. However, even taking back the space currently taken up by the ODS and the crew compartment on the Orbiters, you'd be lucky to shoehorn two ISS components into a Shuttle C, let alone three. It depends on the relative sizes, of course (two nodes, sure, but not two truss segments), but Shuttle C's volume is the limiting factor here rather than the payload capacity.

Weight and volume, okay, I can see that. Its too bad because 6 orbiters and 6 shuttle C was so simple. Yet I can accept that to design a larger payload fairing would subtract from the cost savings.

But at 2x (truss in orbiter and 2 node payloads in shuttle C) then its 8 + 8, right? 16 fewer orbiter flights and 16 less chances that disaster strikes. If the payload is 2 ISS components and not 3, then might a two engine version be sufficient and thus cheaper?

I don't think I'd try to cost justify Shuttle C with ISS completion only. Ops costs should be radically reduced (just from eight or ten Orbiter prep cycles being eliminated), but I'm not sure that even eight missions would get you to break-even when DDT&E and facilitization are figured in. If it cost you $2B to develop and facilitize for Shuttle C, that cost alone would make the per-launch cost at least $250M -- add onto that the cost of SRB turnaround and $50M for an external tank and the operations costs to stack and launch the thing.

I'm just speculating here, though, as NASA's accounting practices for such things are hardly my area of expertise. Maybe they *could* come up with some formulation by which it makes economic sense, but I don't really see it.

In practical terms, however, I'd guess Shuttle C for ISS is not in the cards for schedule reasons. ISS is supposed to be completed by 2010, and even if the Shuttle C design could be picked up where it left off in 1992-1993 and taken to completion, it would be lucky to reach operation in 2008-2009.