Bulk manage Google Contacts with dual SIM card
Accidentally stumbled upon a useable solution. In the Settings menu on the top is a menu item 'Manage SIM". This is meant for setting up names and defaults for the SIM cards. At te very bottom there is a menu item 'Link contacts' (translated from the dutch 'Contact verbinden'. 
Next step is to select the SIM card and then you can select contacts to link to the selected SIM card. (Don't forget to press the 'Add contact' icon at the top-right after selecting the correct contacts, instead of the checkmark at the top-left.......)