(1) It is indeterminate whether 'Bob' denotes Bob1 or Bob2.
(2) It is likewise indeterminate whether 'Mirror-Bob' denotes Bob1 or Bob2.
(3) But it is determinate that 'Bob' and 'Mirror-Bob' do not co-refer.
After all, if you took 1 and 2 to be the fundamental semantic facts in this scenario, then there is nothing to rule out their being coreferential -- it would simply appear to be indeterminate whether that's actually the case. To supervaluate: there are four ways* to resolve the indeterminacy, and on two of them the names co-refer, and on the other two they don't. So we cannot settle the question whether the terms co-refer.
* = Those four ways are:
(a) 'Bob' denotes Bob1, 'Mirror-Bob' denotes Bob1 [co-refer]
(b) 'Bob' denotes Bob1, 'Mirror-Bob' denotes Bob2
(c) 'Bob' denotes Bob2, 'Mirror-Bob' denotes Bob1
(d) 'Bob' denotes Bob2, 'Mirror-Bob' denotes Bob2 [co-refer]
But that's the wrong way to go about things. While 1 and 2 offer a partial description of the semantic situation, such an atomistic approach cannot capture everything that's going on. It is whole scenarios that are instead fundamental. Compare 1 and 2 with:
# (4) It is indeterminate which of the four semantic scenarios, a-d, obtains.
This is the situation as Alex took it to be. But it is not what I had in mind. And this way of presenting things brings out some alternative possibilities, such as:
# (5) It is indeterminate whether semantic scenario a or d obtains.
(6) It is indeterminate whether semantic scenario b or c obtains.
This is my claim. Crucially, it is 6, not 1 and 2, which provides the fundamental account of the situation I had in mind. (1 and 2 are consistent with any of 4, 5, or 6. That is why they are merely partial descriptions.) To derive more particular claims -- e.g. my 1, 2, and 3 -- from the fundamental account (6), we simply supervaluate. That is, a claim is:
(I) determinately true if it is true in all allowed scenarios;
(II) determinately false if it is false in all allowed scenarios; and
(III) indeterminate if it is true in some allowed scenarios and false in others.
Since my allowed scenarios are b and c only, we obtain the following results:
My claim 1 is true because 'Bob' denotes different people in scenarios b and c.
Likewise for claim 2 and 'Mirror-Bob'.
3 is true because in both b and c, 'Bob' and 'Mirror-Bob' denote different people.
Does that all make sense?