The thing is you can only use one of the two lines for data since there is no data switching between lines. For instance if you choose your primary line for voice and data, the secondary one will be used only for voice and SMS. If you get a call to your secondary line, you won't have data while on the call unless you are on wifi, since there's no data switching between lines.
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the secondary line is not necessarily voice and SMS only, but it will be if you choose the primary for voice and data. Only one line can be used for data since there's no data switching.