Only if they are different species. Root fusion is a thing and it can help lift young trees up to the canopy by slipping them extra sugars through the root system.

Two vivid demonstrations of this are albino trees that grow despite having no chlorophyll of their own, and living tree stumps - the stump scars over, yet never grows branches of its own, because it is fused to a sibling nearby. I’ve seen pictures of both, but have only seen the latter in person.

There’s a fantastic board game on this theme, for anyone interested in such things.

Players can only grow their plants or spread seeds in spaces that receive light. Plants obscure light, the bigger the plant, the bigger the shadow area. The sun moves around the board each turn. All this means some very simple rules lead to some wonderfully complex emergent gameplay, where you’re trying to time actions against the sun’s movement to maximise light intake, while having to deal with other player’s actions unpredictably altering the patterns of shade.

https://boardgamegeek.com/boardgame/218603/photosynthesis

Same with a lawn. Certain plants like clover have advantages like not requiring much nitrogen. Many grasses are very fussy and need good soil to thrive. But the grass finds its advantage in that it can grow taller than clover and therefore block out the sun. As the grass blade gets longer it is able to collect more sun (surface area grows) and establish deeper roots and block out the clover even better.

Many people use a variety of chemicals to hold weeds at bay. But a well watered lawn that is regularly fortified with organic nitrogen and kept at a decent length will more or less prevent most weeds from establishing. Many people cut their lawn way too short, removing the grasses advantage.