Below is a simple example for three files. Separate groups are created in all three files, but in the third file a button is added to the group of the first file.
1st file:
<customUI xmlns="http://schemas.microsoft.com/office/2006/01/customui"
xmlns:nsWb1="My Shared Tab">
<ribbon
startFromScratch="false">
<tabs >
<tab
idQ="nsWb1:QMySharedTab"
insertBeforeMso="TabHome"
label="My Shared Tab" >
<group
idQ="nsWb1:QGroup1"
label="Group 1"
visible="true">
<button
id="Button1"
label="Button1 in Group1"
visible="true"
onAction="Button1_onAction"/>
</group >
</tab >
</tabs >
</ribbon >
</customUI >
2nd file:
<customUI xmlns="http://schemas.microsoft.com/office/2006/01/customui"
xmlns:nsWb2="My Shared Tab">
<ribbon
startFromScratch="false">
<tabs >
<tab
idQ="nsWb2:QMySharedTab"
insertBeforeMso="TabHome"
label="My Shared Tab" >
<group
idQ="nsWb2:QGroup2"
label="Group 2 from Wkb2"
visible="true">
<button
id="Button2"
label="Button2 in Group2"
visible="true"
onAction="Button2_onAction"/>
</group >
</tab >
</tabs >
</ribbon >
</customUI >
3th file:
<customUI xmlns="http://schemas.microsoft.com/office/2006/01/customui"
xmlns:nsWb3="My Shared Tab">
<ribbon
startFromScratch="false">
<tabs >
<tab
idQ="nsWb3:QMySharedTab"
insertBeforeMso="TabHome"
label="My Shared Tab" >
<group
idQ="nsWb3:QGroup3"
label="Group 3 from Wkb3"
visible="true">
<button
id="Button3"
label="Button3 in Group3"
visible="true"
onAction="Button3_onAction"/>
</group >
<group
idQ="nsWb3:QGroup1"
label="Group 1"
visible="true">
<button
id="Button4"
label="Button 4 from Wkb3"
visible="true"
onAction="Button4_onAction"/>
</group >
</tab >
</tabs >
</ribbon >
</customUI >