So, the difference between dual standby and dual active dual SIM Android phones are:
- No difference when the phone is in standby mode.
- When you are using one of the SIM cards for voice calling (or sending SMS) through 2G network, the other SIM card will be disabled in dual standby phones.
In this sense, dual standby usually is referred as dual standby only, or dual-standby-single-active.
When using 3G or 4G mobile data on one SIM card on a dual standby phone, the other SIM card will NOT be disabled. 3G and 4G LTE mobile data connection always uses packet switching, which does not require exclusive usage of the receiver. Although 2G (or 2.5G as named by some carriers) mobile data
GPRS (General Packet Radio Service) and
Edge (Enhanced Data Rates for GSM Evolution) actually are both packet based. But due to the implementation, exclusive use of the receiver is usually needed.
If you want to make both SIM cards always available in a dual standby phone, you need to set up call forwarding and enable call waiting as explained in
How to make two SIM cards always available in dual SIM dual standby Android phones page.