Stealth (penetration and survivability), speed (imparting energy and time to target), payload (volume and weight), Agility.
Four parameters to compromise. J-36 compromises out payload and some agility. J-50 compromises out payload to a greater degree. H-20 would compromise out speed and agility. JH-xx or non-stealth supersonic bomber as Beijing Hammer 2.0 would compromise out agility and stealth.
There's definitely other attributes to having a two platforms. It's a workaround on cost and powerplant limits to only a slight degree. No aircraft has all four in spades. B-21 compromises out payload (relative to B-2 H-20 and JH-xx), agility, and speed.
5th parameter is cost but J-36 is probably the only aircraft in the world that gets closest to the highest coverage. It has all of them to some decent degree but more payload than most fighters, similar speed, higher stealth, lower agility.
A hypothetical complementing bomber that is less stealthy or non-stealthy but supersonic and can carry at least 2 ALBMs, ALHGV, ALHCM would be also useful in other ways that H-20 isn't. Time to target and imparting energy. Having those two platforms simultaneously means all three major categories are covered (no bomber is going to be agile).