@james-blythe My motorcycle friend has a TomTom, and I have a Garmin. The way we enter the (shared) routes in our device(s), is to import the track into the device, not the route. Doing it this way already several years now, we have hardly ever had differences. Indeed, differences mostly (only) occur when on one of both devices doesn't have a navigable road where the other one does have one.
This way of working (i.e. using the track rather than the original route) is less labour-intensive than the one explained above by Hans above (and I may be wrong, but I think there's a smaller chance of resulting into different routes). On the other hand, it has one disadvantage which may be your show-stopper : you lose the POI's of your original route(s). If you want to have the POI's available in the routes, you have to do it the way Hans explains.
But irrespective of the way you put the route(s) in your devices : to me, riding the same route with a TomTom and a Garmin together is combining the best of the 2 worlds, we hardly ever have problems finding a way around any event on our route (like deviations, road works, or the likes).