Plants like animals are competitive organisms.
As they mature they will compete with each other for resources( space, water, nutrients, light etc.) Chances of losing one or more because of that are not insignificant. If not, speed of growth and health might be impeded.
In the long run, the trunks might grow against each other, splay outward and create deformities/weak spots that might fail down the line.
Short lived species like your Alnus (Betula is another) MIGHT be worth planting in this way since they'll most likely be petering out anyway by the time issues might arise (if they survive each other's competition that is).
But I concur with John on the matter of seeing this in nature.
