Alien Gear Shapeshift for multiple guns

Hi all, depending on the weather and some other factors I carry either a Glock 30 or a Glock 42. I'm looking at the Alien Gear Shapeshift, specifically the appendixcarry option. But when I was pricing them out I noticed something odd.While I can buy a holster for either gun I cannot select one gun and then add the shell of the other gun to the order.In other words I can set the 42 as my main gun but I cannot then choose the 30 as an option under "Additional Shell".The same is true if I choose the 30 as the main gun. Then I cannot choose the 42.So what I'm asking is this: will a 30 shell fit on the backing that is designed for a 42? (Or will a 42 shell fit on the backing designed for a 30?). Or are they preventing me from adding the other shell specifically because they are incompatible? via /r/CCW https://ift.tt/2NHcYUn