There are several ways to address this, but the easiest is to execute SSJS that refers to the target data source by name. For example:
<xp:panel>
<xp:this.data>
<xp:dominoDocument var="secondDataSource" form="someForm" />
</xp:this.data>
<xp:button value="Save Second Data Source" id="button1">
<xp:eventHandler event="onclick" submit="true"
refreshMode="complete">
<xp:this.action><![CDATA[#{javascript:secondDataSource.save();}]]></xp:this.action>
</xp:eventHandler>
</xp:button>
</xp:panel>
This should only cause the named data source to save. If you make the save event a partial refresh targeting just the panel that contains the target data source, and set the event to partial execution mode, that should skip validation for components bound to the other data source.