What combo of knives should I get

[Ourorboros]

Well suggestions have already been made, so I'll just point out that for boxes you want a slicey blade. Something thin.
Cardboard can really wear at an edge though so you need something to sharpen your blade.

Since some of this is for work, I'd pick up a box cutter to go with my "real" knife. Which one depends on how often I open boxes. Like a Home Depot folding utility knife is fine for a box here and there, but a Milwaukee Fastback utility knife has a fatter handle better for harder use.

 

[TechandTactical]

I personally prefer to carry a decent knife and a multi tool. Much more flexibility than 2 knives.

 

[pinnah]

I personally prefer to carry a decent knife and a multi tool. Much more flexibility than 2 knives.

That's where I've ended up. I pair a Leatherman Micra in the right front pocket with a rotating large traditional folder in my right rear pocket (held upright by a seam I've sewn in all my pants).

OP, for an SAK, I would look first at the 2 layer designs. I find that too much width makes a knife uncomfortable for me to carry in my front pocket. I can go much longer and heavier, so long as the profile is nice and flat (e.g. Buck 500 Duke carries better than a lighter but fatter SAK).

The Leatherman Sketatool might fit your needs.

Whatever you get for smaller knife/tool, I recommend a cap lifter as a "must-have" tool.