You could also do a cross join with a where condition similar to your inner join condition
var q1 = from billMap in tblOfferingBillingBehaviorMapping
from lkpBill in tblLookUpBillingBehavior
where billMap.LkpBillingBehaviorId == lkpBill.LkpBillingBehaviorId || billMap.LkpBillingBehaviorId =lkpBill.ParentRootId
select new { billMap, lkpBill };
solved Whats is Linq equivalent for SQL query with OR-joins? [duplicate]