If destination is below mins in conditional section (TEMPO, BECMG, ETC) 2nd Alternate required regardless of 1st alternate derived minimums: could be severe clear and you would still need a 2nd, because you used 3585 to dispatch. If destination is below 1-2-3 rule, but not below established mins in any section, you only need one alternate and would only require a second alternate if 1st alternate was below derived mins in the conditional section. Why not just use 2nd alternate in that case; most of the time you would. But if you service the 1st Alternate city, but not the 2nd ...then you will likely see both alternates.
NOTE: there are conditional limitations and derived limitations that deal with visibility and/or ceilings that must be considered.