As you can see in the link to the docuentation
https://www.odoo.com/documentation/user/9.0/purchase/purchases/tender/manage_multiple_offers.html
there is written that you can create a purchase tender and after save and confirming you can choose multiple vednors to get the request for quotation.
But i tried it in the online demo and also on my own odoo 11.0 this option isnt available.
Or do i something miss?