If you want 2 lesser subs right away then get the NSDs. But if you want better bass in the long run then I'd get a single ULS-15 or F15 & get a second when you can.
I've heard a single ULS-15 & a dual ULS-15 setup. Both were awesome!! Of course the dual setup was better but even the single ULS blew me away.
I still think that a Quad ULS-15 setup would be one of my all time sub systems. That or an I.B Manifold with 4-18" FI drivers...mmmm ok sorry back to topic!!
So I forget what the recommended enclosure dimensions are for the TC LMS5400 18". So if that's not a viable option for your space then I'd shoot for dual 15" (HSU/Rythmik).
I'd be worried you'd get the dual 12"s & later wish you held out for dual 15"s with that large of a space.