After extensive investigation on discussion boards, I now understand that a dual SIM phone's IMSI or SIM serial numbers cannot be found (except for contacting the manufacturer).
My original query has been amended to: Can we even tell the phone has two SIMs?
I think someone with some intellect could find it.
I can think of a few ways, including:
tracing the logs for the IMEI number and dialling a USSD code (I tested this in India with *139#).
This will provide me with the SIM's IMEI number from which I called the USSD code.
(The phone is assumed to adhere to Android standards and have two IMEI numbers.)
preserving the SIM's IMSI and/or serial number. Additionally, even if the phone was not rebooted (i.e., the SIM was replaced), any additional IMSI/Serial numbers could be found by tracing particular logs or by handling broadcast events.
You may view both IMEI numbers by dialling *06#. Get those two digits somehow. (Something like to screen recording and text image parsing.)
Anyone with more ideas is encouraged to share them. Any form of assistance would be greatly appreciated in this case. Additionally, if anyone is aware of any manufacturers' APIs or connections to contact them, please do share that information with the community members.