Up to you. If it was my house, I reckon I wouldn't want redundant equipment going rusty behind the walls.
Could you sell the back boiler as scrap metal? Could it be left in the front garden with a sign on it saying 'FREE'?
As others have suggested, a redundant chimney should at least be capped. Alternatively the stack should be taken down and the hole made good by extending the rafters and covering with roof tiles, otherwise you will be spending money on maintaining a redundant chimney stack / flashing / pointing / brickwork / flaunching, from now on.