PhD: When to quit and move on?
This can really only be answered by you. But here are some thoughts. 

There are two doors. One leads to a doctorate and the other does not. If there is no path to the first door, then the decision should be clear. But there are a couple of possible paths to that first door. 

One is to move elsewhere or get a different advisor. If that path isn't closed to you then you should be considering it. Expensive and time consuming, yes. But whether it is worth it is up to you. 

Another, possible, path is to cut the old prof out of the picture and work exclusively with the younger one on some idea and just finalize it without consulting the older prof. 

A third, is to cut out the younger prof and work exclusively with the older one and his group, perhaps getting some support from them and hopefully more from the old professor. 

But it seems like you are being whipsawed. Find a path that works. Even if it is to the 'no degree' door. But make sure that you will like what is on the other side of that door before stepping through it. 

My personal preference (maybe not yours) would be to work with only the younger prof. But get a plan agreeable to both of you that if you follow that path you will find success in a reasonable amount of time. I mean, preferably, a written plan, or at least one that clearly lays out the parameters. 

If you don't have a plan, you probably don't have a path. 



My situation was a bit different, though also difficult, but I switched universities after four years in grad school and found a much better situation and an excellent and helpful advisor. My future seemed bleak at the time, but it was with the encouragement and help of another professor that got me situated.