Not 100% sure about the relationship between these two entities but here goes:
IList<Splitting> res = (from s in [data source]
where s.Customer.CompanyID == [companyID] &&
s.CustomerID == [customerID]
select s).ToList();
IList<Splitting> res = [data source].Splittings.Where(
x => x.Customer.CompanyID == [companyID] &&
x.CustomerID == [customerID]).ToList();