you could use an interface and/or a super class (abstract or concrete), depending on the similarity between your 2 classes)
Code:
<class name="IAgreement" table="AGREEMENT" discriminator-value="A">
<id name="id" column="AGREEMENT_ID">
<generator class="native"/>
</id>
<discriminator column="DISC" type="string"/>
<subclass name="YourFirstAgreementClass" discriminator-value="AGR">
<property .... />
</subclass>
<subclass name="YourOtherAgreementClass" discriminator-value="otherDiscrimnatorValue"/>
<property .... />
</subclass>
</class>
just make your two classes implement the IAgreement interface or Agreement super-class