On the LG smartphone, a single contact can be assigned to a SIM card. Regardless of which SIM card is defined as the default, the contact will use the registered SIM card. The desired SIM card can be selected under the Cost-saving mode setting.

Android 9 | LG UX 9.0
Step 1:  Open the  Contacts -App
Step 2:  Tap on desired  Contact
Step 3:  Tap on  Edit
Step 4:  Tap on  Cost save mode
Step 5:  Choose desired  SIM card
Step 6:  Tap on  Save
  1. Open the Contacts-App
  2. Tap on desired Contact
  3. Tap on Edit
  4. Tap on Cost save mode
  5. Choose desired SIM card
  6. Tap on Save

LG Instructions