A friend of mine and I used to link up to work for each other on occasions.
We were both sole traders. We would work together on each other's work. Our agreement was that money wouldn't change hands, but that we would work to maximize each other's earnings.
It worked well as we only did this occasionally and there was no hard and fast commitment, so if anything were to go wrong we wouldn't end up in a wrangle and fall out. So it wasn't a merger as such, just a loose arrangement to give each other a boost.
John