If either target creature is an illegal target by the time Combine Guildmage’s second ability tries to resolve, nothing happens. No creature loses or gets a +1/+1 counter.

If a creature would normally enter the battlefield with no +1/+1 counters on it, Combine Guildmage’s first ability causes it to enter the battlefield with a +1/+1 counter on it.

If the first target creature for Combine Guildmage’s second ability has no +1/+1 counters on it as the ability resolves, nothing happens. The second target creature doesn’t get a +1/+1 counter.