What you need is validationGroup property
The required Filed Validator will have following property
<asp:requiredfieldvalidator id="RequiredFieldValidator2"
...
validationgroup="Group1"
...
runat="Server">
</asp:requiredfieldvalidator>
And associated button
<asp:button id="Button1"
text="Validate"
causesvalidation="true"
validationgroup="Group1"
runat="Server" />
In this way you can organise which validation to perform on any specific button click