[color=red]1-I have to help them more than he has.
[color=indigo]2-I must help them more than he must.
Could these sentences ever mean:
a-I am under a bigger obligation to help them than he is.
Or do they just mean:
b-I have to give them greater help than he has.
Difficult to say without context.
With no context, they are more likely to mean b, but there’s no reason why they couldn’t mean a in certain circumstances. (though I don’t like the second “must” in your sentence - how about “I must help them more than he does.” - that makes it sound like more of a competition or obligation.