I have a wired planet bike computer I got for $29 that I do this with. I bought a second handle bar cradle mount and sensor unit from planet bike direct for my second bike ($10). The computer has settings for bike 1 and 2 so you can put in different wheel sizes for each bike if they are not the same. Just have to remember to change setting from 1 to 2 when you switch bikes. Same cradle and sensor on each bike makes it real simple to go back and forth and still accumulate total mileage... This gives you a complete solution with dual sensors and mounts for $40 bucks. Just have to move computer over and switch bike settings.