How to sister 2x4s on top of each other?
A lot depends on what you need these 2x4s to support - how much weight, what kinds of stresses. Supporting a storage area/attic is different from supporting the first floor of a 2-story house. Doing this for a 12' room is different from 50' across a whole house. Lots of variables, and this may well venture into &quot;consult an engineer&quot; territory.
But in theory, yes they could work - metal plates or plywood.
One important thing to keep in mind: a 2x8 is actually 1.5x7.25. a 2x4 is 1.5x3.5. So 2 2x4 is 1.5x7, which is 1/4&quot; shorter. That may or may not matter for your intended use, particularly depending on whether this is replacing or extending existing 2x8s vs. new construction.