OnMultiNewRound gets called on every round_start even in L4D2, which means twice in a versus round.
Using BOTH is unwise and will only lead to confusion, particularly if you want something random to happen.
Then, youre spawning the wrong shit. weapon_first_aid_kit_spawn is a medkit "spawning entity". Once a new round starts, the Director will fill it as he sees fit (which usually means spawning all meds he can). You do not want to create those 'Spawners', but medkits themselves.
Alternatively, create a code that wipes existing medkits in these spawns on a new round, and then randomly forces new spawns to happen.