@Philfy I doubt that it's possible.
But nowadays you don't save your contacts on a SIM but to Google account. Then you can choose the SIM to make the call.
Other alternative is to save them to Xiaomi Account. But for me Google account is the preferred way, so you can keep your contacts when you switch to another phone brand.
Doesn't matter wherever your contacts are saved.
The Xiaomi's Dialer will merge them in one contact list. Then you can chose the SIM Card 1 or 2 before call. (annex)
Already tested.