Tokens don't go on the stack, only spells and abilities. [mtg:Gather the Townsfolk] creates both tokens simultaneously.

1. You cast [mtg:Gather the Townsfolk].
1. Gather the Townsfolk resolves:
    1. Put two 1/1 white Human creature tokens onto the battlefield. [mtg:Cathars' Crusade]'s triggers twice.
1. Cathars' Crusade's ability is placed on the stack twice.
1. The second instance of Cathars' Crusade's ability resolves:
    1. Put a +1/+1 counter on each creature you control.
1. The first instance of Cathars' Crusade's ability resolves:
    1. Put a +1/+1 counter on each creature you control.

Both tokens (and any other creatures you might have had) got two +1/+1 counters.

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As for your second question, someone casting a split second spell would not change anything.

A split second spell between steps 1 and 2 would resolve before step 2, then you continue with step 2. Same goes for one cast between steps 3 and 4, or between steps 4 and 5.

Split second merely stop you from casting a spell or activating an ability in response to the split second spell.