It isn’t the same. Extra legs are a deformity.
Multiple tree trunks are a form of branching to get more sunlight and water so more leaves produce chlorophyll to feed the tree. It isn’t a deformity; it is a matter of increasing chances of survival.
And it isn’t all that rare. Redwoods will form a number of trunks from a single bole; they will grow in a circle.
Aspens have an interconnected root system. One aspen tree is actually only a small part of a larger organism. A stand or group of aspen trees is considered a singular organism with the main life force underground in the extensive root system. Before a single aspen trunk appears above the surface, the root system may lie dormant for many years until the conditions are just right, including sufficient sunlight.
And, there is a fungus in Oregon that is essentially a single organism that covers over 2,200 acres