There's no easy answer to this question - it depends on a number of factors, including the type of coach you're on, the route you're taking and the facilities available. In general, however, it is possible to have a poo on a coach - you just need to be prepared for the possibility that there may not be anywhere to dispose of it properly.
If you're on a long journey, it's always best to try and go before you get on the coach. If you know you won't be able to make it to the end of the journey without going, then take some toilet paper or wipes with you so that you can clean up afterwards. It's also worth bearing in mind that coaches can sometimes be quite cramped, so if you're feeling at all uncomfortable about doing your business in public then it might be best to hold off until you can find a more private place.
Of course, there are always exceptions to the rule - if there's an emergency situation or if nature really calls then obviously you'll have to do what you need to do! In these cases, just use your common sense and clean up as best as you can afterwards. Nobody wants to sit in someone else's poo, so make sure any mess is cleared up properly.
So there you have it - everything