select 
iclient0_.Id as Id0_, 
iclient0_1_.CompanyName as CompanyN2_1_, 
iclient0_2_.Surname as Surname2_, 
iclient0_2_.Firstname as Firstname2_, 
iclient0_1_.CompanyName as formula0_, 
iclient0_2_.Surname + ' ' + iclient0_2_.Firstname as formula1_, 
iclient0_.ClientType as ClientType0_ 

from Client iclient0_ 
left outer join Company iclient0_1_ on iclient0_.Id=iclient0_1_.Id 
left outer join Human iclient0_2_ on iclient0_.Id=iclient0_2_.Id 

where iclient0_1_.Surname + ' ' + iclient0_1_.Firstname=@p0
-- А нужно 
-- where (iclient0_2_.Surname + ' ' + iclient0_2_.Firstname=@p0) or (iclient0_1_.CompanyName=@p0)

    public interface IClient
    {
        Guid Id { get; set; }
        string Caption { get; set; }
    }

    public class Human : IClient
    {
        public virtual Guid Id { get; set; }

        public virtual string Caption { get { return Surname + " " + Firstname; } set { } }

        public virtual string Surname { get; set; }
        public virtual string Firstname { get; set; }
    }

    public class Company : IClient
    {
        public virtual Guid Id { get; set; }

        public virtual string Caption { get { return CompanyName; } set { } }

        public virtual string CompanyName { get; set; }
    }

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
                   assembly="testSubclassWhere"
                   namespace="testSubclassWhere.Domain">
  <class name="IClient"
         abstract="true"
         table="Client">
    <id name="Id" column="Id">
      <generator class="guid"/>
    </id>
    <discriminator column="ClientType" type="string"/>
    <subclass name="Company" discriminator-value="C">
      <join table="Company">
        <key column="Id"/>
        <property name="CompanyName"/>
        <property name="Caption" formula="CompanyName"/>
      </join>
    </subclass>
    <subclass name="Human" discriminator-value="H">
      <join table="Human">
        <key column="Id"/>
        <property name="Surname"/>
        <property name="Firstname"/>
        <property name="Caption" formula="Surname + ' ' + Firstname"/>
      </join>
    </subclass>
  </class>
</hibernate-mapping>