There is a gemara (Pesachim 110) describing the concern for ruach ra with eating even numbers of foods, in particular:
- 109b - Question: Why did Chachamim enact to drink four cups? This is dangerous! i. Beraisa: One should not eat two (or any even number of) things, drink two cups[...]
- Rav Dimi: A tradition from Moshe from Sinai teaches that one should be concerned for two eggs, two nuts, two gourds and two of another matter. Chachamim were unsure what the other matter is, therefore they decreed about everything.
The gemara presents different rejoinders from the Sages of Blessed Memory explaining why the four cups are okay on Pesach. But the only responses that would seem to apply also to the lechem mishnah, the "double bread" (the two loaves we bless on Shabbos and Yom Tov and at all festive meals, I think) are Rav Asi's Berachos 51b that "we do not bless Birkas ha'Mazon on a cup of punishments" (i.e., that the blessing/mitzvah aspect means we are unassailable by default) and the Tosfos that "ruach ra is not frequent among us." Neither seems to be a full explanation of why lechem mishneh is okay.
Is this discussed anywhere? (I am also reminded of the fact that the ur- lechem mishneh, the showbreads [and, @DoubleAA notes, the double manna], were in an even number - 12  - and that we have 12 strands total in most lechem mishneh at the Shabbos table.)