That is the setup I have on my boat. Originally the runners ran back to the winches but didn't like it because it took a winch so I put in two tackles sized about the same as my vang. I am not sure they are big enough though. I don't think the mast would come down if one broke but I would not be able to point very well under just the stay sail without the runners. They are not original. The mast was designed without them but the previous owner thought it pumped too much. I had the rig completely retuned after I bought it and I don't get any pump. I think he just had everything too loose. The runners are good insurance though.
There is a tide in the affairs of men,
Which, taken at the flood, leads on to fortune;
Omitted, all the voyage of their life
Is bound in shallows and in miseries.
Shakespeare, Julius Caesar IV, iii, 217