Well, there are "only 2 of us," too but since I'm one of them we've never had a "regular sized tri-tip" (2-3 pounds) last long enough to freeze. With dinner, sandwiches, tortilla-wrapped Mexican things, other snacks and such the leftovers are gone in 2-3 days.
I occasionally vac-pac about a pound but again, it keeps long enough in the fridge. I've never frozen cooked tri-tip so not sure how well that turns out and pretty much have convinced SWMBO that it just wouldn't be "right."
In either event, I agree with the Aggie - "Cowboy up" (or, in your case, "Bear down") and cook it all at once. It's only 3 pounds and as good as it is as leftovers, lighting up a smoker or grill for half doesn't seem worth the trouble (unless you've cut it into steaks I guess, but that has never worked nearly as well as a whole roast for me).
Jus' sayin'...