If two challahs are braided and baked together on one pan, and before they are completely baked, they have stuck together, do I have one challah or two? i.e. can I separate them and still have two challahs for lechem mishneh?
The same question would apply to store-bought challah rolls that come in a six pack where all six are stuck together.
You have two challot.
R Simcha Bunim Cohen writes in The radiance of Shabbos (p. 77)
Loaves that become attached during baking may be separated and are still considered whole for the purpose of lechem mishne
In the footnote he refers to Machatzit Hashekel 174:1
Similarly Shmirat Shabbat KeHilchata 55:4:6 writes
Two loaves which became stuck together whilst being baked may be used as lechem mishneh, even if they are still attached to each other.
However there is a view which disputes this and holds that one must first separate them.
In the event that one separates them from each other, and they remain whole, they are indeed considered whole loaves, even if one can tell they were joined together. Where part of one loaf remains stuck to the other, it is well to remove the piece, so that the loaf to which it is stuck should indeed look whole.