Once in a while, two or more players want to cooporate in doing something. For example lifting a heavy object.

I do not think I have seen any decent rules for this. The only assist rule I have found is for maneuvers, which adds a Fortune dice. This is really lackluster so I have tried to come up with some other way for someone to really assist in the action.

How do you handle these situations?

You could upgrade to a green die.

I dunno. That still seems kind of weak. And it does not take into account the stats of the assisting character. For lifting something, for example, a ST4 character should be a lot more help than a St2 character.

One solution is obviously to just have each character roll the test and then add the successes together, but that leads to a lot of rolling and it does not give a enough of a feel of cooperation.

I guess I will probably just continue to wing it

There are rules in Hero's Call for multiple characters to add to a dice pool. You might look at them and be inspired by them for joint tasks.

In my experience, nearly anything done outside of combat has at least two other PCs providing an assist, and even just the official team-up dice can add up quickly. Success rates are pretty high in this game as-is. If you bump assists up much above +2 white per helper, no one will ever fail at anything. Not trying to talk you out of it, just encouraging you to run the math on whatever house rules you implement.

In EotE the assisting rules go as follow

To build up the dice pool Take the highest stat from any of the participating characters and take the highest skill from any of the participating characters. If one of the helping characters is just worse than the others in everything, then he only adds a boost (fortune) die to the dice pool.

All this goes accompanied with the typical text saying that the players have to describe how their characters help each other bla bla bla.