Let take a shot at asking the question you actually asked...

Yes, I've spliced two computer heads into a single speed sensor wire. It worked, but was problematic since you create a single point failure for both computer heads, i.e., if your pick-up sensor or the wheel magnet get knocked out of alignment you loose both computers. You must also make sure the splices are sealed up so that no moisture can get in to the splices, so shrink wrap is a must. The computers don't know that they're running off the same signal wire, so no issues there. I believe the computers I spliced together were Trek and Sigma Sports models, with descent wire that could be soldered. Not all computer wires like to be spliced, so you'd need to Google on "slicing Vetta Computer wires" or something like that to be sure.

Ultimately, I ended up running pick-up wires for each computer off the front fork: ganged together on one fork so that one magnet would trip both of them. The stoker's wires ran down her boom, around my seat post and along the underside of my top tube, pretty much the same route that you'd use for a splice. On some tandems that don't have super-long stoker compartments you can usually run a rear-wheel sensor wire to the seat stay / brake block area along the underside of the stoker's top tube. I did an article with pictures on this technique about 11 years ago: http://www.thetandemlink.com/articles/bc1200.html

As others have noted, wireless is quickly becoming the range where cost is not a big consideration, noting several folks will have dual Garmin units, etc.. (ourselves included). However, wired-computers still dominate the landscape on many tandems, moreso that the small sampling here might suggest.