I haven't tested this, but I think you can do the following:
1. Dial party number one (using a Speed Dial, if desired).
2. Hookflash, placing the first call on hold and getting a new dialtone.
3. Dial party number two (using a Speed Dial, if desired).
4. Hookflash, conferencing the two calls together.
Either or both of the two calls could be placed using trunks on the local OBi or (with the appropriate configuration) on a remote OBi.
This thread describes how to configure two OBi's to be able to call out through each other:
Single-Stage Dialing Through Any OBi Trunk