I'm wondering if I can use use some bread that is partially sliced (but not completely severed in two) as lechem mishneh. An example is a bagel that was cut 90% of the way through, meaning that the two halves are still attached on one end. Is this permissible? Is it less than ideal?
Shulchan Aruch 167:1 rules that during the week one should start to cut the piece he wishes to eat from the loaf before reciting hamotzi. However, one needs to ensure that it remains connected enough so that if he were to lift up the piece, the rest of the loaf would come with it. Mishnah Berurah 167:6 explains that if one were to cut the piece to a greater extent, the loaf would lose its status as a complete loaf.
Rema ad loc. notes that on shabbat one should not cut a piece until after reciting hamotzi. (Mishnah Berurah 167:10 explains that this is due to a worry that one will cut the piece too much, to the extent that the loaf is no longer considered whole.)
However, if one forgets and cuts it before hamotzi as he does during the week, Rema writes that he has not done any harm.
Mishnah Berurah 167:11 explains that in such a case he still fulfills lechem mishneh because
שכל שאוחז בפרוסה ושאר הככר עולה עמו הוי כשלם
as long as one can hold the piece and have the rest of the loaf come up with it, it is considered whole.
So, ideally, one should use a totally uncut loaf on shabbat. However, as long as the partially detached piece is attached enough so that one could pick it up without losing the rest of the loaf from it, one fulfills the requirement of lechem mishneh.