Ok Sant, I think this is what I've got and what I can get:
My issue actually was in a ListView, but I still think it would apply.
I ended up with the following working XAML
<gridviewcolumn header="Gravity Model ID" displaymemberbinding="{Binding Path=GravityModel.Definition.IDGravityModel}" /><gridviewcolumn header="Gravity Model">
<gridviewcolumn.celltemplate>
<datatemplate>
<combobox name="cbxGravityModels" itemssource="{Binding Path=GravityModels}">
SelectedValuePath="Definition.IDGravityModel"
SelectedValue="{Binding Path=GravityModel.Definition.IDGravityModel}"
DisplayMemberPath="Definition.Name">
</combobox>
</datatemplate>
</gridviewcolumn.celltemplate>
</gridviewcolumn>
I think part of you problem could be the second datatemplate - or more likely as you suggested: The problem may lie in using the same source for two different comboboxes one of which is not in the grid template leading to a selection in one reflecting in them all.
This is my shot - hope you sort it out.
Cheers
Erik