You can do what you want. Any number dialed on the OBi100 will be routed out the OBi200's SP3. Speed dials must be modified to work. This setup uses the OBiTalk network to route calls between the boxes, therefore the OBiTalk network must be functioning. There is a more complex setup that doesn't use the OBiTalk network. I'll show you how if you want.
Use OBi Expert to make the following changes:
OBi200
Voice Services -> SP3 Service -> MaxSessions: 4
Voice Services -> SP3 Service -> X_InboundCallRoute:
ph,pp(200222222)
Voice Services -> OBiTalk Service -> InboundCallRoute:
{>0:ph},{200222222:sp3},{ph}
Replace 200222222 with the OBi number of the OBi100
OBi100
Physical Interfaces -> Phone Port -> OutboundCallRoute:
The 1st rule is: {([1-9]x?*(Mpli)):pp}
Change it to: {(<510111111*>(Mpli)):pp}
Replace 510111111 with OBi number of the OBi200
Physical Interfaces -> Phone Port -> PrimaryLine: SP1 Service
Voice Services -> OBiTalk Service -> InboundCallRoute: ph
Edit:
Dialing zero will ring the OBi200.