It depends. If the touch in is at a gate then it will be fine. If the touch in is at another validator then it may be treated as an exit. If the whole journey you eventually make is covered by your travelcard then there won’t be an issue. If travelling beyond the zones covered and both starting stations have validators and are within the zones covered by a travelcard then it might be safer to touch in on a bus (and get straight off). This will ensure that the second touch in is treated as a touch in and buses are always free with any travelcard.