Every two flute endmill I've ever bought has been center cutting, and the description has never called it out. I've no idea why the Enco rep quoted above said they weren't—i have a cabinet full of end mills it in the garage, mostly from Enco, that says it's clearly not true in sizes up to and including 3/8". Bigger ones night be a different story.
Rough it out in steps with an endmill smaller than the groove width, and then use a long endmill for a finish pass. You can get reduced shank end mills in the larger sizes, but blending the sides of the groove together will be very hard.
Sent from my Nexus 6 using Tapatalk