I notice you are also including Köln in your itinerary. Köln is in the German state of Nordrhein-Westfalen. It's not in Rheinland-Pfalz or in the VRM. At €33, the Schöner-Tag-Ticket, the Länder ticket for NR-W, is the most expensive of the Länder tickets.
If this travel is on a weekend day, use a Schönes-Wochenende-Ticket for €37 for both of you. It covers travel throughout all of the Länder.
If this is a workday, unless you are doing a lot of travel in Köln before you go to Koblenz, you won't want to get a Schöner-Tag-Ticket. Get the Rheinland-Pfalz ticket from a red DB automat in Köln and point-point tickets to Bonn (Bonn is a border station for the RL-P ticket).
The point-point fare from Köln is €6,60 pP to Bonn and €9,80 pP to Remagen (inside the VRM). It will cost you €39,60 for the Köln-Remagen-Minigruppenkarte combination vs €41,20 for the Köln-Bonn-RL-P ticket combination. It will only cost €1,60 more for the Köln to Bonn and Rheinland-Pfalz-Ticket combination and you won't have to stop in Remagen for the Minigruppentageskarte. If you are only going to go to Koblenz Hbf that day, the point-point fare for 2 from Köln to Koblenz is €34,20.
OTOH, if you find you can purchase a VRM Minigruppentageskarte in Köln (unlikely), or you get off in Remagen, you could get the 3-day Minigruppentageskarte, for €40, and save more money.