There was a walnut tree planted within 12 feet of my neighbor's foundation and he claimed it caused a crack. The tree was getting too large and I had to take it down.

But a more important consideration is the health of other plantings, which may be harmed by the juglone produced by the roots. Not all other plants are susceptible, but if there are any of importance to you, you should check this out.

You say you are buying grafted trees, but you don't say what rootstock they are grafted onto. Is it a dwarfing rootstock? I don't know what progess has been made in dwarfing rootstocks for walnuts, but it would go a long way towards solving your problem.

Another idea might be having both varieties grafted onto the same rootstock for a 2 in 1 tree. This would save space. Some people here have advocated planting two trees in the same hole for similar reasons, that it tends to dwarf both trees.

Thank you very much for info.
I do not know what rootstock they are grafted onto.
Here is Grimo nursery site;
http://www.grimonut.com/catalog.php?catID=59

Contact them and ask. I would not advise you going ahead with this w/o the information. As far as I'm aware, there aren't dwarfing rootstocks available for walnuts. If full-size, it's recommended to keep them at least 20 ft from a foundation and spaced at 40 ft apart.

Did you also see that they don't consider their grafted trees suitable for your zone?

My own experience with walnut is as full-sized shade trees, not for nuts. Hopefully, someone else here will have better information.

Here in eastern North America, most grafted walnuts, regardless of species, are on eastern black walnut - J.nigra - understock; so...any issues attendant to juglone toxicity from J.nigra roots will be the order of the day, despite the 'tops' being J.regia.

yay: does it mean I might be lucky.
Thank you guys