I take my dog on quite a few bike adventures. She is relatively small (less than 40 pounds), so she has options. She can backpack, trailer, or run along with us on mountain bike trails (still in training as she still gets a little deaf when other dogs are around).
How large is the dog? Assuming it is a standard guide dog lab/retriever, the backpack will probably not work, so your best bet is a trailer. We have a Burley Tail Wagon (haha, I love a good pun!) but the weigh limit on it is relatively light at 70 pounds. If you guys are near Falls Church (or could get there), I am happy to let you take ours for a test ride if the pup is under 70 pounds.
If the trailer will not work (due to weight limits or the dog just not wanting to be confined), you can use a bike-canine specific tow leash to attach to the tandem. I would recommend the Bike Tow Leash for this. While we have not tried it ourselves (it is on the list of things to buy and try), I have heard really good things about it. Link for info is here.
Hopefully this helps!