How does Teamwork... work?

It's your own radius. When you put trait points into 'Teamwork' and increase its range, you're extending how far away you can be from your own teammate before no longer receiving the buffs.

I've tested this myself and my range is smaller than my partner. When we begin to separate, I always lose the bonus before him. You can see below your partner's name in the bottom right, a little fist icon that indicates you're receiving the buff for being in range.

Small thread discussing this too.

In this configuration, what happens: does only Player Red get the Teamwork buff, does only Player Blue get the Teamwork buff, or do they both get the Teamwork buff?

Only the blue partner will get the buff.