To book a train ticket with two seat reservations, first start the process as you would for a regular train booking.
After you have entered all the required information and selected the required fares, you will be taken to the "Ticket & Reservation" tab, where additional offers are listed.
Click "Proceed" and Deutsche Bahn will automatically assign you a seat. However, in your case, two seat reservations are needed, so navigate to "Ticket & Reservation" again.
There you may change the number of seats using the drop-down menu.