Consider the following query. Customer is a field that allows multiple SPUser
<Query>
<Where>
<Or>
<Contains>
<FieldRef Name='Customer' />
<Value Type='UserMulti'>foo</Value>
</Contains>
<Eq>
<FieldRef Name='Requestor' LookupId='TRUE' />
<Value Type='User'>12</Value>
</Eq>
</Or>
</Where>
</Query>