How to join two 1x6s to form a 23' baseboard with a seamless transition?
The standard practice is a 30 degree scarf joint preferably joined on a stud. I like to miter my two end with the same setting so I know the angle is perfect(when possible) then take my pieces to their wall a little over sized and scribe them to the floor (making sure the top line of the trim is on plane) and mark out for your end cuts. Once fit, position your pieces and join them with glue and micro pins(skip this if you don't have a micro pinner, brads just make a mess). Let it tack up for a minute and the start securing it with finishing nails. Try to crisscross nails at the scarf without getting too close to the joint.  Cheers!