The distance between the two pipes is called pipe centre. One of those pipes will be for hot water feed and should be connected to the radiator using a valve. The other pipe will then be connected to the other side of the radiator where the colder water will return to the system. Please note that pipe centres on all ladder type towel radiators are 50mm shorter than the actual width of the radiator itself. i.e., a 700mm wide and 800mm wide ladder type towel radiators will have 650mm and 750mm wide pipe centres respectively.