If you're concerned that your reservations would not be next to each other, go ahead and book the ticket without reservations. When you board, just walk through the train and look for a pair of empty seats. Go to the next car(s) if necessary, but stay out of first class if you don't have a first class ticket.

Check the electronic reservation display above the seats (or the little card holders on older trains); if it's blank, you can take those seats. If it shows two station names, the seat is reserved between those stations and you can sit there before and/or after. If it says "Expressreservierung – gegebenenfalls freigeben" there's a chance someone may have reserved it after the reservation data was loaded into the train's computer at its originating station. If it says "Bahn Comfort – gegebenenfalls freigeben" it's an unreserved seat for Bahn Comfort members (like preferred elite seats on airlines). Anything unoccupied 15 minutes after the train leaves the station is fair game, but you may need to move if someone boarding at a later station has a reservation.

What's the downside risk? If the train is crowded, you might end up not able to sit together (no worse than if your reservations were for non-adjacent seats). If the train is totally packed, you might even have to stand until a seat becomes available, and good luck finding two together in that case. However, for high traffic routes/days/times, the booking system will say "please reserve" to alert you to that possibility, in which case it might be best to go ahead and get reservations and then switch on the train if a seat next to one of you is open.