Welcome! I agree with Mateo & Alok’s input and would add that it can be tricky for a package colony to reach a full buildout of two deeps in one season. It’s still pretty early tho and maybe it’ll be a nice long flow. As Mateo said, keep an eye on how far along the first box is as the weeks progress.
You could adjust your base for stability by placing those cinder blocks on end, with two next to each other and the other pair a few feet apart from those to allow two sturdy lengths of wood to fit into the top holes on each side. Just make sure the planks are level and your hive will be more stable & at a better height for you while still keeping critters from bothering it.
Edit: here’s a post about single vs double brood box info you might like to read: