Create a component which will allow to control which command to initiate form submit when enter key is pressed.

For example such component will allow to execute button#2 by default when a user hit Enter:

<h:form id="form">
...
    <h:commandButton value="Ajax button" id="button1" actionListener="#{sampleBean.ajaxSubmit}">
         <f:ajax execute="@this" render="@form">
    <h:commandButton>
    <h:commandButton value="Regular Button" id="button2" action="#{sampleBean.nonAjaxSubmit}" />

    <ace:defaultAction target="#{someEL_poinitng_to_button2}" />
...
</h:form>

The component could also support more keys in addition to Enter.

Example:
<h:form id="form">
...
<f:ajax execute="@this" render="@form">
    <h:commandButton value="Submit" id="button1" actionListener="#{sampleBean.ajaxSubmit}"/>
    <h:commandButton value="Cancel" id="button2" action="#{sampleBean.nonAjaxSubmit}" />
<f:ajax/>

    <ace:defaultAction target="button1" key="Enter"/>
    <ace:defaultAction target="button2" key="Esc"/>
...
</h:form>