Length and structure
Portal 2's co-op campaign runs to a handful of evenings, and community-made chambers on PC extend it considerably afterwards. It Takes Two is a substantially longer single arc - a proper campaign with a beginning, middle, and end that most pairs finish across a few weekends.
If your pair wants something with a definite ending you will remember, It Takes Two is built for that. If you want something to dip into cheaply and finish quickly, Portal 2 is.
Platforms and how you play together
Portal 2 is same-platform co-op and is effectively a PC game today - that is where it is cheapest and best supported, and where Remote Play Together turns split-screen into online play at no extra cost.
It Takes Two is on PlayStation, Xbox, PC, and Switch, with split-screen throughout and online play using the same split presentation. It also has no crossplay, so both of you need the same platform family. Our split-screen page covers the same-room option for both.
Which one for which pair
Two people who like puzzles. Portal 2, easily. Puzzle co-op is a small category and this is the best thing in it.
A couple looking for a shared story. It Takes Two. It is about cooperation as a theme as well as a mechanic, which is why it heads our co-op games for couples page.
A pair with almost no budget. Portal 2.
One person who plays a lot and one who does not. It Takes Two. Its asymmetric abilities mean the less experienced player always has a clearly defined job, and there is no fail state to speak of.
Two people in different houses on the same platform.Either - both support online play - but It Takes Two's Friend's Pass makes it a single purchase, which is often the deciding factor.
Do not overlook the rest of what you are buying
Portal 2 includes an entire single-player campaign alongside the co-op one, and it is one of the most celebrated games ever made. That is a lot of value in a package most people buy for the co-op alone.
It Takes Two is co-op only - there is no solo mode - so if the two of you stop playing together, the purchase stops being useful. Worth knowing before you buy it for someone as a gift; our gift guide covers that trap.
The obvious third option
If your pair is on different platforms, neither of these works and you should buy Split Fiction instead - same studio as It Takes Two, same two-player design, plus crossplay and a Friend's Pass. Our Split Fiction review covers it, and our 2-player page ranks the whole category.
The verdict
Buy Portal 2 first. It costs so little that it is not really a competing purchase - it is an evening you can afford this week, and if your pair enjoys it, you have learned something useful about what you both like. Then buy It Takes Two when you want the big one, and remember that only one of you has to pay for it. Full write-ups are in our Portal 2 review and It Takes Two review.