While both trees will fit in the specified area with room to spare in their current sizes, you have to account for the mature sizes of each. Left to themselves, both plants will eventually become quite large in size. Most cultivars of Japanese Maple are known to get at least 6x6, and both, even if pruned, will still grow larger with time. A 6x6 space can contain one of the trees, with pruning as necessary to keep it within the dimensions of the space as the tree grows larger. I would say plant it a minimum 3-4 feet from the house and preferably 6+ feet from the maple. Otherwise, they will crowd each other.