About a year ago I converted an older Santana tandem from 7 speed to 8 speed. I had a wheel that used an 8'speed freewheel, so that was not an issue. I was able to spread the rear triangle out some and the new wheel fit fine. I have ridden 8 speed free wheels on a tandem for thousands of miles so unless you are 300+ pounds I would not worry about the axle/wheel. I used a new Ultegra rear dérailleur and 8 speed bar end shifters, it worked great. I have since sold the bike and upgraded to a new tandem with 10 speeds that work great and are not silly.
Wayne