Two of us are staying in Koblenz Sept. 21-24,2018. Should we rent bikes in advance or wait till we get there and go thru a bike shop? Also, is it better to take train to destination then rent bikes in the town vs. in Koblenz? Just planning on biking easy 10-13 miles each day for 2 days. One day the Rhine, next the Moselle. Any suggestions/itinerary of where to start and end? Planning on using trains, boats and bikes! What is the best way to communicate if you suggest renting in advance?
Here is a start.
https://regioradler.de/pages/en_en/start.php?lang=EN
Electric bikes have really become popular in Germany. You might consider them. Some restaurants, hotels, bike shops, camp grounds etc. offer battery charging mostly for free. If you haven't experienced an E-Bike they are great. You always have the wind behind your back (tailwind). They would make it easier to get out of the valleys too and up into the vineyards, castles etc.
Search for E-Bike verleih (rental - Mosel/Rhein) and E-Bike Ladestationen (charging stations- Mosel/Rhein) There is a lot of information and firms that rent and offer tours.
You might be surprised at how little of these rivers you will see on a bike if you're doing 10-12 miles per day. Koblenz to Lahnstein and back is 12 miles.
Below is a list of bike rental shops along the MOSEL River listed by town (Alf, Alken, etc.)
The Bullay/Alf/Zell/Pünderich/Reil area is a beautiful one for biking: https://www.zellerland.de/fileadmin/user_upload/rad_4_10x5.jpg
The ferry (Fähre) operation in Alf (across the river from Bullay) rents bikes: https://upload.wikimedia.org/wikipedia/commons/d/dd/3_6_Bullay_%28Hintergrind%29_F%C3%A4hre_nach_Alf.JPG
From Koblenz I'd probably take the train to Bullay and rent there or in Alf; return to Koblenz by train once your other sightseeing is done (Cochem?) by train.
Some apartments and hotels have bikes on hand for rent or for free. This place I stayed in St. Goar a few years ago does:
https://www.vacation-in-rhineland-palatinate.com/holiday/1611/index.htm
Some places in and around Cochem offer a free pass to guests for local train rides to other towns on the Mosel River:
https://www.elfriede-fuhrmann.de/gaesteticket-cochemzell-englisch.htm
Perhaps you could find a place to stay in one of the small Mosel or Rhine towns where these goodies are available (and where the atmosphere is more bucolic and less modern-urban-citified.)
I like the Rhine very much but for biking the Mosel tends to be better. Much of the Rhine biking tends to be adjacent to the main road (very busy) and the train tracks - see photo: http://www.bahnbilder.de/1200/111-001-4-mit-dem-dbm-803030.jpg - and hence quite noisy and distracting.