We have new construction with tongue & groove interior pine walls. I put a pickled oak finish (2 coats) on the walls & one coat of Hellsman Spar Urethane w/UV protection. I have been advised to use to coats of the Urethane. Is it necessary to use 2 coats of Urethane, since I have the pickled oak on there also?

Yes you need two coats of the spar urethane as the stain does not help to protect the wood and give you a long lasting durable finish. You will need to sand the first coat of urethane to get a smooth finish as I am sure it raised the grain of the wood and it only needs to be fine or extra fine sandpaper or sanding sponge.

Hope this helps out.

PS.
Be sure to buy a good quality chinese bristle brush to apply the urethane.