Correction, make that "coping" cut with a coping saw.

I avoid having both ends coped since it means you have to hit the length perfectly.

My customers would be pretty ticked off with an extra scarf joint.

If you have a helper to assist with the measuring, it's better.
I always leave the last 16" of an open crown un-nailed, so I can tap the joint closed. Even if you have the right cope, if the other side of the corner is not oriented to match, it will taper open. Tapping the joint more down the wall or more up the ceiling draws the joint together perfectly.
Usually I back off 1/16 from the actual measurement, but for a double-ended cope probably best to leave that 1/16 for a tight snap-in fit.
In actuality, the longer the run, the better you will fare with your double cope. It's really hard to get a 2' one to fit perfectly, because there's no room to spring it into place.
Casey

Thanks, it sound like my idea of splitting the last wall run into two pieces has no buyers/support.

My problems include lack of a strong helper, my wife can help hold a length up while I attach it. I do not have a pneumatic (or other) nailer so I have to also hold the nail and hammer while holding up my end. This is my first, and likely my last crown molding project. I'm just getting too old for new tricks. I did cut and install with screws a molding backer, just a wedge at 38 degrees, and wide enough to fit the flat back of the molding. So, I can nail anywhere along the length.

I believe the job will work out fine, and no one will notice any joint or splice problems unless they walk along and inspect the job inch-by-inch.

"I do not have a pneumatic (or other) nailer ..."

Get a Paslode that uses butane cartridges.

If it's paint grade you have a little more room because you can use fillers. Done right you'll never see the joints. You always want to strive for clean cuts, joints, dead on work, but with paint grade projects, you have room for minor errors. Sombriels method of cuttin a tad big and snapping in place works. You want the opposing pieces the copes are running into as right on as you can get concerning the angle.

Thanks, this is primed/white and will get semigloss white trim paint. I figured some careful filling and finish sanding at the scarf joints (as well as at the less-than-perfect coped joints) will cover most errors in fit.

It is hot/humid today a good day for some inside (air-conditioned space) work. I'll finish the installation job...I appreciate the good/valuable inputs.

I finally got around to installing the last piece of crown molding, with both ends copped. I took the advice here and did a careful measuring (about 11' 10") and cut one piece of molding with a copped joint on each end. To my surprise the piece fit just fine... "with a little bit of luck..." I really can't claim to be able to do such a difficult cut every try, but his time it worked and thus no joints along this wall, or the wall opposite it (but that run is butt ended at both ends. That is easy to cut exactly..)