Maybe a dumb question, but why does Hornady's 300 gn. XTP Have 2 canalures and the XTP-mag have only one?

The original XTP had 2 cannelures to allow longer seating using the bottom cannelure, thus allowing room for more powder, in those .45 Colt cylinders that were long enough to allow such seating.
Presumably Hornady assumed one cannelure will be adequate for the length all .454 cylinders on the market, for use with the Mag. version. Two vs one also helps to identify each.

Any info on what velocities the mags work at vs. the standard bullets? I looked on their website and didn't see any listing. I bought a box of the Magnum bullets and will load them out of my pistols and my carbine. I am sure that the carbine will have no problems achieving higher velocities.

According to the Hornady 2012 catalog the standard 45cal 300gr XTP is rated 800fps to 1600fps.
The 45 cal 300gr XTP/Mag is rated 1100fps to 2100fps.

I shot resized 300gr 44 serrias resized to 416 out of my taylor at 2650. Had no trouble driving them through whitetails.

Thank you sir for the information.

Looks like I will keep them for my rifle as I can say I would have no problems keeping them above the 1100fps. Although the Lee 300gr RNFP with a gas check is what I shoot most out of all my 45 colt loads.