What happens if the Heliod/Spike Feeder combo meets the Heliod/Walking Ballista combo?
It depends on which creature has more counters

Say the Ballista has 4 counters, the Spike has 2.

Bob activates their Ballista (3 counters left).  While the trigger is on the stack:
Alice activates their Spike (1 counter left).  While the trigger is on the stack:
Bob activates their Ballista (2 counters left).  

Alice cannot activate her Spike again, or it will die and she will have no combo.  She's forced to let Bob resolve his Ballista trigger and his lifegain trigger.

Before Alice's Spike trigger resolves, Bob activates their Ballista again (2 counters left).  

As you can see, Bob's combo is running, and Alice's is stuck.  Bob can kill Alice.

On the other hand, if the Spike had more counters than the Ballista, then Bob will lose his ability to run his combo before Alice does, and Alice will always be able to gain more life than Bob deals.

If the Spike and the Ballista have the same number of counters, whomever starts activating their combo first will lose the race.  If Alice activates her Spike before Bob activates his Ballista, then he can kill her.  But if he activates his Ballista first, Alice will survive.

Of course, the Ballista combo also generates an arbitrarily large sum of life for Bob, so he is motivated to activate it anyways (since Alice already has a large lifetotal).



Notes: Letting the activated ability resolve and then activating your own ability on top of Heliod's triggered ability achieves the same result as if you had interrupted the activated ability.