2 male cats fighting all night - please help?

We adopted a 4 year old male cat from the RSPCA a few months ago and we adopted another one yesterday, an 8 year old male. Both have been neutered. We separated them to start with, letting the new cat roam the room, whilst one of us sat with him and the other sat outside with the old cat roaming the rest of the house. Then we introduced them to each other and they were fine at first, just following each other and smelling. They have separate bowls for food and drink and seemed to be getting on. But last night when we went to bed, they were snarling and hissing at each other. Our old cat had the new cat pinned to the front door and wouldn't let him get out of the way. We tried separating them but it happened again, snarling and hissing. We then put the new cat in the room with the door shut and the old cat wandered the house but he still kept hissing and scratching the door to try and get to the new cat, the new cat was making a really strange noise the rest of the night like a howling noise. We didn't get any sleep and both cats were still not happy this morning with each other

Has anyone else had this with there cats? Or does anyone know what we can do to help the cats? Have read various stories on the net that 2 male cats can't live in the same house but the RSPCA didn't say anything about this to us so not sure if this is true.

Thank you