I used to carry 2 phones...
Tried to find a dual SIM phone with dual standby mode and HSPA for both SIMs... no chance.
I decided to go for the Skyroam Gmate.
Now I'm running a unlimited phone call/sms/data SIM + simple SIM in Gmate.
It runs great. It allows me to have both numbers active at the same time.
For now, this is the solution (for me).
Sent from my SAMSUNG-SGH-I927R using XDA