Found this in my yard. One type of leaf is needle-like and about 2 1/2" long with vertical veining. The other is like a mimosa or honeylocust with compound bi-pinate leaves. They're growing from the same stalk!!
There are a couple of terms that will be useful to you in your researches here: dimorphism and phyllode. Dimorphism is simply the occurrence of more than one leaf type on a single plant, which is more common than you might think; take junipers for example where there can be sharp juvenile leaves and blunt mature leaves on one bush. Your example is related to the phyllode and is common on Acacia species.
The image above is a smaller version of an image from Wikimedia commons of Acacia koa, a tree from Hawaii. You don't say where you are from or what sort of plant it occurs on, but these would be common in places like Australia.