In a 4 stroke engine, oil is stored separately, pumped into various parts as the engine starts and does its lubrication and cleaning work and returns to the tank. After every 5,000 KMs or so, the oil along with all the dirt is drained off. However, in a 2 stroke engine, generally the oil is mixed with petrol and directly taken to the combstion chamber. There is no re-circulation here and the used oil is thrown off into the atmosphere. The ability to clean the engines is known as 'detergency' and both the oils need it. However, 4 stroke oils also need 'dispersancy', the ability to hold the particles of combustion in suspension. For this purpose, special polar additives are added into 4 stroke oils, known as dispersants. They are essentially nitrogenous compounds.
A 4 Stroke oil is hence a more complete product than a 2 stroke oil. No useful purpose is served by adding a 2 stroke oil as a top-up to a 4 stroke oil. Well-formulated engine oils are tailor-made for each type of engine - 4 stroke or 2 stroke - and each type of fuel - petrol or diesel.