Do you have a converted Glock? Because I do, and with the conversion barrel, the reliability just isn't there. Yes, for a 'range toy', it's probably fine.....but what's the point of carrying .40 and shooting 9mm at the range? All you're doing is:
a) taking the cost of a pistol, and adding a conversion barrel and different magazines - cheaper than buying 2 guns
b) not practicing with what you actually carry - i shoot a few types of firearms and w/ practice i am proficient w/ them all
c) having to stockpile 2 different calibers of ammo when you could just have one - stock up on what you shoot the most and if .40 is for defense, keep a smaller amount of defensive ammo
Again, there is NO real compelling reason to do what the OP wants to do. A straight up 9mm will be a much better choice.