Check whether an array can be the result of merging two smaller arrays, maintaining order
I placed the if condition for checking lengths at the top even before the index definitions because I figured that if they aren't even the same size, why bother with doing anything else. Is that weird?


No I don't think that is weird. I would consider it a good optimization.

Personally I think the code is mostly fine, a good starting point would be to Google some examples on how to merge sub-arrays since that is fairly similar.

My one question would be why you started at the end of the array and merged them in descending order. That is kind of strange to me. I would do it in ascending order. I would also use a for loop instead of a while loop.