On our Co-Motion Speedster Tandem we have Ultegra brake/shifters. In preperation for a trip I bought extra cables in case of a problem. Though the problem now is I cannot see how to get to the derailer cable! I can see how to access the brake cable after I use a jeweler's size phillips but am clueless about the shifter cable. I looked at Shimanos site but the exploded view did not seem to help.

Which model of Ultegra do you have? I'm guessing the 6700 family? If so, the cable entry port is under the rubber hood. You shift the lever to the small-cog position, peel the hood forward, and push the inner wire to feed it backwards out of the lever. There's also an "escape hatch" secured by tiny Phillips screws that you can use if the head breaks off the cable and needs to be extracted.

As for the model? I am not sure how to determine that. Is there a number stamped somewhere?

Peel the rubber hood forward and you'll find the model number molded into the plastic lever body. The 6700 family is the latest generation and hides the shift housings under the handlebar tape, for an easy way to identify them visually. They also require removing the front trim cap to change the brake cable, and you mentioned using a screwdriver to get at the brake cable, so I deduced you probably have 6700 (6703 if it's a triple crank).

Assuming you do indeed have the 6700-series brifters, here is Shimano's tech document showing their installation and set up. The cable routing is shown:

https://techdocs.shimano.com/media/te...9830702876.pdf

The earlier ones you just need to click down to the bottom cog/chainring, pull the brake lever a little and the cable should be able to slide out of the little hole that's now aligned.