In essence it's a single grain long burn. For manufacturing and shipping it done in several smaller grains.
If you fly it as is the grains will burn end to end and from the core out. Increasing the pressure and decreasing the burn time.
So by gluing them together you are making a single grain motor again, that burn from the core out only.
Think of the J-90, K185 etc., they are all 1 grain long burns with an offset slot.
You should use a dowel to keep the grains aligned while doing this, you don't have to, but it makes it easier. Don't be stingy with the epoxy, make sure to coat the grains liberally to get a good seal.
When you glue the grains together, you don't want to get too much epoxy in the hole, just the end faces. Covering too much of the core will result in a low initial thrust. If you follow the instructions, you will probably get enough epoxy in the liner to glue the grains to the liner. The epoxy inhibits the grain ends so the grain burns as a 1 piece grain. If it were to be shipped as a 1 piece grain, it would have to be shipped as explosives 1.3C instead of 1.4C. (Limit is about 3 lb. per grain.)