Hi Stefan,

See these codes :

class product_template(osv.osv):
    _name = "product.template"
    _inherit = ['mail.thread']
    _description = "Product Template"

class product_product(osv.osv):
    _name = "product.product"
    _description = "Product"
    _inherits = {'product.template': 'product_tmpl_id'}
    _inherit = ['mail.thread']
    _order = 'default_code,name_template'

product.product inherits product.template so you should add you new field in product.product. Your new field will available to both.