What I ended up doing was to manually trigger my validation when form B was submitted and transition to a decision-state that checks if there were validation errors. It's a little ugly, but I feel like it's the best way:
<view-state id="start" model="compositeForm">
<transition on="formAsubmit" to="pageA" validate="true"/>
<transition on="formBsubmit" to="isFormBValid" validate="false">
<evaluate expression="formBValidator.validate(compositeForm.formB, messageContext)"/>
</transition
</view-state>
<decision-state id="isFormBValid">
<if test="messageContext.hasErrorMessages()" then="start" else="pageB"/>
</decision-state>