I'm sorry, I've been trying this for over an hour but I can't get them to align. Any tips? Thank you!!
For this box shape, the easiest way to ensure that the texture on the faces are aligned after unwrapping is to mark seams in such a way that it unwrapps as a single UV island.
Then rotate and scale in the UV space as needed :
If you do have UV islands floating about that need to be aligned then use the snapping tool to align them. (and some rotations etc). Note that if the scale of the two islands are different then you may need to snap individual vertice or edges into position.