Securely joining 2 4x4 pieces of wood
Slice them up into 1x4s then laminate them with a 1 1/2 foot overlap on alternating pieces.



That will give you a 6 foot 4x4 from two 3 foot 4x4s.

You'll need lots of wood glue, many clamps and either a good table saw or something equivalent.

The only other ways I can think of doing this involve spending more on hardware than a 6 foot 4x4 probably costs.