are staying in St Goar 2 nights before heading on to Trier. Is there a discounted ticket for this route[?]
Not a discounted ticket as such, but if you buy an RMV (local Frankfurt metro) ticket to Bingen Hbf for €8,70/P, and a Bahn ticket from Bingen to St Goar for €7,50/P (Total €16,20/P, €32,40 for both of you), you'll pay less than the €44,20 for both of you (€22,10/P) for Bahn tickets from FRA to St Goar.
Next day, two Bahn tickets from St. Goar to Bingen Stadtbhf will cost €30,40 (€15,20/P) vs €27,50 for two (€13,75/P) if you buy 2 single trip Bahn ticket from St Goar to Bacharach for €4,75/P and 2 RMV tickets for Bacharach to Bingen Stadtbhf. This might not be worth the savings if you would have to get off the hourly train from St Goar to Bacharach to buy the RMV tickets and get back on the next train to Bingen Stadt in an hour. Alternatively, you should be able to buy the Bacharach-to-Bingen Stadt tickets from the automat at St Goar or you could buy them at the Regionalbhf at FRA before you leave.
I say Bingen Stadtbhf because the closest train station to the KD dock is Bingen Stadtbahnhof (not Bingen Hbf). At the Stadtbhf, take the underpass under the tracks and turn left up the ramp to the street, then turn right and go across the street and to the walkway next to the river. Turn left and walk 200 meters along the river to the KD ticket kiosk. By the way, show them your Bahn tickets for a 20% discount.
The third day, for St Goar to Trier, a 2 person Rheinland-Pfalz-Ticket, for €31,- will be less than 2 Bahn one-way tickets St Goar to Trier for €66,40 (€33,20/P).