Lord Sessadore
Explorer
That could definitely be one solution. It really doesn't matter, as far as I can see, until and unless tempests are approved. For tempests, without double weapons, you need to use a smaller [W] weapon to get your bonus damage from Tempest Technique, so it doesn't make sense to me that they should be able to spend one feat to get both a defensive weapon and a larger [W] than any other off-hand weapon.If it were "off hand" only to offset the oversized off hand weapon penalty that afflicts anyone who isn't a 2W ranger... other than that, it doesn't get the benefit of being an off-hand weapon.
Anyway, giving them a sort of pseudo-off-hand property that merely lets you wield the double weapon, but doesn't provide the other benefits of an off-hand weapon seems fine to me.