Hi,
Go to Tcode: RSPLAN
Click on Info provider tab :Give the info provider name and click on edit, you can see the Characteristic relationship tab.
1. Click on Create button.
2. Select the Char.relation type as "Attribute"
3. Select the Attribute name.
4. Check attributes.
4. Click on save button.
For the second question yes you need to have both char A and B in query.
For more information about CR please find the below link.
https://help.sap.com/saphelp_nw73/helpdata/en/4c/b82d6b54182102e10000000a42189e/content.htm
Best Regards,
Madhu