Two 3k's would give you two power supplies and two power cords, and double the fans/heatsinks for cooling, and quad 2-ohm stability (and maybe double the caps if you are lucky with the right amp.)
But also twice the rack space and fan noise. No free lunch.
If you are measuring below 10hz, I'd get the CSL UMIK-1 which is calibrated flat to 5hz instead of 10hz.
I'd say 2 18's in the space would be the minimum, dual 24's and a fp14k would be better, but also significantly more money...
Each 24 has about the same output as ~3 18's, give or take. It's much more compact, much louder/deeper and less distortion.
I don't think it would be wise to limit the mid-bass to 200watts, that's gonna severely limit your SPL.
You are sacrificing all >20hz for more <20hz, you'll have no mid-bass, given you'll be limited to 2 subwoofers and likely a concrete floor, that probably isn't wise.
(But that's sort-of how I started off, gotta start somewhere, so who am I to say no...
No only will the SPL be low above 20hz, but you'll lose 10db in the first 10ft due to distance-law.
So your dual 18's are gonna sound like dual 6's. (Not really, but you know what I'm saying...)
I wouldn't take the "start cheap and re-do later" approach. In the long-term you are much better off just saving up more money and going directly for better products from the start. Doing otherwise is the approach that is wasting money.
I think you'd be much happier with dual $500 B&C 21's, or dual $200 B&C 18's and solving the upper-bass firstly (20-300hz), and worrying about <20hz later/never.
If space is tight then maybe a single B&C 18/21 and a single SI-24 powered with the FP14k and miniDSP as an alternative.
The only downside I see with that is the unbalanced sound pressure at different frequencies, left side of room vs right side of room from the different woofers. They almost need to be placed behind the seating or under the center channel and close together, so that they sound like 1 extremely loud sub.
I still think it would be better to have a bit of unbalanced sound with authority, then weak 200w upper-bass into already inefficient drivers. The B&C's or PA-460's is like 98db/w/m vs 89db/w/m, that 10db difference is hard to overcome (is never overtaken).
Either that or: just model the box size to allow the full power into both UM-18's, and let the infrasonics be what it is.
Then the >20hz bass (which is the most important bass), will have full SPL at least. 1000w x2 or whatever it does.
As you are probably aware, spreading subs out causes nulls to form at some given frequencies relative to the distance between them, but it provides smoother bass throughout the rest of the room, but can sometimes be localizable; where as placing them together makes it achieve closer to the +6db "ideal" summing and thus a bit louder, but more severe peaks/nulls (depending on placement), and even worse localizability.
Physics and money is always working against us... (never with.)
Everyone has to pick their customizable-poison eventually... (and then they are stuck having to sleep with those decisions forever. The bed is made.)