My understanding (which is not great) is that the universe is expanding.
Imagine two airport style people movers moving in opposite directions. They mimic space expanding.
Then imagine two people on the people movers moving at the speed of light in opposite directions from one another.
They would end up farther away from each other then the distance light travels times 2 because of the expanding nature of space.
I’m not saying I necessarily believe that but that is the answer you would be given.
Would that be the same concept as two masses approaching each other as an opposite? Like say two vehicles each traveling 60 Mph towards each other which would make the speed of an impact a true combined speed of 120 mph?