I don't think you'll be able to get away without a formula. The only exception to that requirement would be if you distilled from a mash (of grains), which is typically what an oude genever would be. We wanted to use that phrase on our label, but of course we couldn't, even though the oude refers to a style, and not any indication of age.
You should be able to get formula approval, and COLA, as a gin for what you describe, since you're essentially making two gins and combining them. It's not too far off from a jonge genever style.