This is a rather..
Insidious rule, as it hides under the frenzy section, but it (sort of) answers the question: (page 298) "However, a comrade that comes to assist with a melee weapon exposes themself to the same frenzy, and
mobs with more than one mass can easily split attacks". No further explanation is given, but presumably, for example, a Mass 8 mob can attack two takers in range, with +3 damage on each attack.
Under Casualties in combat on p. 297 it says that when a Mobs Shamble reaches 1, they attack the character they just reached.
The way the start of "Casualties in Combat" is phrased is very misleading, but actually, mobs will first attack the round after they reach shambles 1. See page 297, at the top: With shambles 3, a mob will attack at the end of third round. Round 1: Move to shambles 2. Round 2: Move to shambles 1. Round 3: Attack. The Example Casualty Encounter on page 300 also supports this.
I think it's unfortunate that the rulebook uses "just reached" on page 297, as it apparently doesn't refer to the same round the movement happened, but the round before. In some sense it's true the mob just reached shambles 1, but still, rather confusing choice of words.