Christian,

you can do it in 2 ways:

1)  As u have 2 seperate views, u can pass some unique context from fields in both views, and in search method use these context values to filter your data..
<field name="<m20_field>" context = "{'view': 'quotation'}"/> in view of Quotation &,

<field name="<m20_field>" context = "{'view': 'sale_order'}"/> in view of Sale Order

then in search method of that m2o_field refresncing object:

def search(self, args, ......):
     if self.env.context is None:

         self.env.context = {}

     if self.env.context.get('view', False) == "quotation":

         #Perform you operation for quotation view

     elif self.env.context.get('view', False) == "sale_order":

         #Perform you operation for sale order view

2) pass domain in the field to filter the values:

<field name="m20_field" domain ="[(<YOUR DOMAIN>)]" />

Hope it helps!

Thanks Pawan. Your answer is useful but not exactly what I need as I just realized my question wasn't complete.

I have 2 menus: Quotation and Sale Order. they call 2 separate tree views.

When I click on create in both forms, always the same form view is called.

In the form view I want to have a field (order type which I created as many2one) to different filters based on which tree forms calls it.