A bit more testing with F5122m Dual SIM + µSD:

Pinout and 4 test cases
1 case: All pins except 4 (V+) and 6 (GND) taped:
-Both SIMs detected
2 case: All pins except 2 ( Card Detect), 4 (V+), 5 (CLK) and 6 (GND) taped:
-Both SIMs detected
3 case: All pins except 2 ( Card Detect), 3 (Command response), 4 (V+), 5 (CLK) and 6 (GND) taped:
-Both SIMS detected
4 case: All pins except 2 ( Card Detect), 3 (Command response), 4 (V+), 5 (CLK), 6 (GND) and 7 (DAT0) taped:
-Only 1st SIM detected and SDcard, too (of course with error message)

So, looks for me that it is not hardware thing (I am not an expert in electronics, just an engineer) as the card is detected and 2nd SIM disabled when there is some data connection (DAT0 in this test) to the SDcard.
Is there some part from original Xperia X software, taking care of this, I really don't know.

Maybe someone with more expertise can go further...