List<JUNC__c> lstToBeInserted = new List<JUNC__c>();
for(XYZ__c A: [select id,f1__c,f2__c from XYZ__c where recordType.name ='A']){
     for(XYZ__c B: [select id,f1__c,f2__c from XYZ__c where recordType.name ='B']){
        if(A.f1==B.f1 && A.f2==B.f2){
           lstToBeInserted.add(new JUNC__c(A__c=A.id,B__c=B.id));
        }
     }
}
if(lstToBeInserted.size>0)
   insert lstToBeInserted;