Can I join two 4x4 fence posts?
Is it possible, yes. Is it practical (.vs. just buying the right size posts), not really. Should it be below ground - no.
To really do it right you want a router bit or shaper cutter that will put tapered &quot;fingers&quot; on the end of the joint which interlock for maximum glued surface - which, unsurprisingly, is exactly what you'll see in the end joints of engineered lumber - but if making a 4x4 from engineered lumber, there would be 2 offset 2x4's that were end jointed, so that the end joints were not all at the same point on the resulting timber.
You can, of course, do it to a lower standard. It may work well enough, or it may fail.
Another option would be to:
Set steel posts and bolt the wood to them. Either cut them off or just let the posts stick 2 feet above the main fence (bolt them to the steel posts above ground level - don't half-bury them.) Depending on the purpose of the fence those extra 2 feet can be useful, i.e to string trellis wire for plants or monfilimaent to annoy deer above your solid fence.