the issue about 2 ppl sharing internet was that they can't play together, but since that is no issue for them, don't know why this is even mentioned. They can't host for the same reason everyone else. Ports.
How to fix?
DMZ.
Although, dmz can only be enabled to one device ip (computer, tablet, console, etc..) on the modem/router.
But since they are in the same house, thats no issue as they can manage who does the hosting
Also why don't you encourage those 2 to come forth on the forum instead of having an intermediary speak for them, they would learn more if they are proactively looking for their own solution.