On Dual-SIM phones I know you will not run into trouble because there is only one 3G data connection at a time. Some devices use the second SIM card only for 2G/phone connections.
If there is really a phone that supports two 3G data connections at the same time the system must allow the user to specify how the connections are used. The common way would be to set routes so that connections to certain IP addresses are routed through SIM 1 or 2. Alternatively each app would be bound to a certain SIM card/connection.
In both cases you don't have to care about this in your app and you have no way to influence it.