Hi amal.jose@nestgroup.net,
With the code from the document, you can change the mux of ADC0 Channels. If you make a small change in the code, you can add the Channel 13, and modify the g_ADC_mux array.

With this, you have the Channel 12 in PTB2 and the Channel 13 in PTB3, both Channels of ADC0.

Hope this helps. If you have any question please let me know.
Best Regards,
Ricardo