Yes, that is a typical set-up for an OBi device.
I have Google Chat / Google Voice set up as Service Provider 1, and my Callcentric VoIP line set up as SP 2 (you can reverse that order if you like). The Callcentric number was ported over from Vonage, which was ported over from my original PSTN telco (GTE, which became Verizon). I have SP 1 set as default, so my outbound calls go over Google, unless I press **2, which sends them over Callcentric. Inbound calls from either SP 1 or 2 ring the same phones, with a "ring-ring" distinctive ring tone for SP 2. This is the default behavior, and is very easy to set up.
Callcentric has very low-cost DIDs (inbound phone numbers), no contracts, and your choice of per-minute or unlimited outbound calling. They also have E911. Note, that if you set up your VoIP provider as SP 2, you should change your outbound call route to force 911 calls to SP 2; otherwise, set up VoIP as SP 1, and it will work with no modification.
{([1-9]x?*(Mpli)):pp},{911:sp2},{(<#:>):li},{**0:aa},{***:aa2},{(<**1:>(Msp1)):sp1},{(<**2:>(Msp2)):sp2},{(<**8:>(Mli)):li},{(<**9:>(Mpp)):pp},{(Mpli):pli}