I think reviews for these Team-Up cards can heavily depend on the situation you plan to use them in.
1 .Colossus hero + Shadowcat ally
- This card is awkward with Colossus, as he is usually fine to take a hit due to his omnipresent tough cards. In fact, due to Iron Will, he might even prefer to lose a tough card. Shadowcat is a below average ally and not a priority in the early turns when setting up for Colossus. If this somehow prevented a piercing attack from discarding tough cards, it might have more use, but unfortunately it doesn't work that way. There are too many downsides for my taste to play this in a solo game, but it is a nice way to redirect an attack against another hero in multiplayer.
2 .Shadowcat hero + Colossus ally
- I like the Colossus ally a lot, so I think the requirement is pretty reasonable. There is awkwardness again in that Shadowcat already has the ability to prevent all damage from an attack built into her form upgrade. So I am not thrilled to play this when Defiance or a basic defend would suffice. You aren't always protection though, and sometimes you are in Solid form when you would like to defend. The best case scenario remains when you can redirect an attack against a different hero.
3 .Colossus + Shadowcat are both identities
- This is the case when the Team-Up condition should be easiest to meet, but remember both heroes must be in hero form and you can never play their ally cards. Additionally, because Shadow and Steel is used as a defense, you won't even know you both need to be in hero form before you draw this card at the end of the player phase. Sure you can hold on to it until next turn, but you're losing value somewhere if you don't use your entire hand every turn.
Final thoughts: A strong card but hard to find the perfect situation for it outside of a multiplayer game. Slightly better with Shadowcat than with Colossus if playing solo. Jocasta is a must-have in protection to save this for the perfect moment.