Hello Pascal,
Loading several QWeb templates is possible with both options, please see:
* https://github.com/odoo/odoo/blob/8.0/addons/mail/__openerp__.py#L74
* https://github.com/odoo/odoo/blob/8.0/addons/board/__openerp__.py#L41
But I don't think it's possible to load different templates for the same Widget and have them available at the same time.
Widgets declare its template only once, the 'PosWidget' declares its template here:
* https://github.com/odoo/odoo/blob/8.0/addons/point_of_sale/static/src/js/widgets.js#L935
For other widgets, as the form widgets, one can create new widgets by inheriting existing ones and overriding their definition (template reference included), and then it can reference the new widgets to be used instead of the existing/default ones.
Even though you can create new PosWidgets, based in the original one, I don't think there is a way to switch widget implementations at runtime.
If someone else knows a way, please let us know.
Regards.