i wonder if the two party thing has an inevitability to it in terms of each side trying to form the largest coalition possible
Do you mean in America? If so, clearly.
Case in point: when the Green Party was just starting to get enough traction through grass roots sweat equity and fending off numerous challenges legally by the Dem party to assure a ballot line in enough states to at least pressure the DNC to move an inch to the left, they (The DNC) created and funded the creation of the Working Families Party.
This left of center-right party was aggressively
Pushing voter registration drives on college campuses. If you saw their employees, you might naturally presume them idealistic young lefties (they looked the part) almost none though were the least bit idealistic.
I learned from them through personal conversations they were all in paid positions.