There are bike computers (speedo/odo etc) that are designed to work with two or more bikes with different wheel sizes, etc. But a cheap used android phone with some sort of cheap external battery booster may work well. I've tried the GPS on an old Android, and what I have is a separate $12 bike computer for each bike and since they both sit in the shed together, I can quickly add the two totals together. But over the years, that has been less important to me as is the total days riding.