Yes, it is very simple by setting your two OBi devices as your circle of trust (COT). For instance
OBi 1 (softphone) : speed dial 1
OBi A (US) : speed dial 2
OBi B (HK) : speed dial 3
When you are in the US, pick your the phone attached to OBi A, key in the following sequence
3 * xxxx yyyy
where xxxx yyyy is the HK 8-digit telephone number. You may press the hash key (#) to make the call immediately.
Please make the PSTN line is the default line (primary link/trunk) of the OBi B. A PSTN fixed line is attached to one of the HK PSTN telco.