Combining the bike trip with a family visit might be worth taking an extra day or two off.

It's sort of like how commuting by bike is actually more 'efficient' time wise even if it's slower than driving/busing because you're combining your fun/exercise time with necessary travel time.

See it as getting to accomplish two goals at once.

Traveling that kind of distance is also awesome from a bike advocacy point of view, if you're into that.

So +1 on the take two days thing. If your wife can handle 50 miles solo, she can almost certainly handle 75 on the tandem. Just pick a nice place to stop for the night.

I'm at a much lower point on the practicality curve. My wife just started commuting a few kilometers to work this year, her longest ride has been in the 35km range. I'm hoping to do 100k or so in a day with her by the end of the year (easy pace with lots of stops).