Oh boy! GO SLOW!
They are a relativley simple kit,,but if you don't do step 1 before rushing to step 5 things will not turn out as nice as they could.
No, there is not a "weakness" issue in the two piece stock. NO, you don't glue the pieces together.
It's hard friend, but you have to do each step at a time, beginning with making the parts fit the wood,
Then sanding the stock to shape, that's right, it isn't in it's final shape ready to be sanded a litte bit then stained.
How much experiance do you have with wood working and/or carving? Several members here will be willing to walk you through it, if you will take the time to wait out the responces, you just posted so give'm just a few day's to find it and you'll have daily posts on the topic.
begin; take a cloth with some kind of oil solvent and wipe down all the metal parts, Barrel, trigger, the lock,,get all that shipping grease off so you don't make a mess with everything you touch. Then try fitting the lock in it's mortise, don't force it, try the trigger in it's mortise,. These parts don't just slide in do they? You need to remove just a little wood so these parts fit the hole. If not, when you finish the wood these holes will swell and be even harder too fit.
Check out how the butt plate fits, see how it's tipped a bit and you can see light between the plate and the wood?? That needs to fit long before you apply any stain.
Take a good look at all the parts and read the instructions a couple more times, check back in here in a little while OK?