I might consider lazily transferring a nearly new, lightly used chain from one bike to another bike once, e.g., to avoid the deep cleaning regiment required to prepare a chain for the initial wax. However, for long term usage, I would keep the chain(s) for each bike separate. Even if both bikes use the exact same chain length, the chain may wear differently on each bike, and I would prefer the drivetrain components of each bike to wear together, rather than moving a chain back and forth and risk introducing strange wear patterns in the drivetrain of either bike.