Well if you're american... I guess the combo should count as an upgrade and then Promise #6 (1 year trade-up policy) should apply -- although it's even simpler since you don't have to ship back your first SB12.
If SVS is ready to ship you two SB12 for $1149, I don't see why they would refuse selling you one for $500 after you just bought one for $649. I'd say odds are good that you could get it.
In my case 1) I bought my SB12 from sonicboomaudio (canadian distributor) so the trade-up policy doesn't apply and 2) I can't justify putting $500 more this year on my home theater