try to set for both (rule 0 and 1):
set 0,1 limit-at=0/512000 max-limit=0/1024000 interface=local_interface queue=sfq
disable 2,3,4
this will limit download only. it should work like this - when both are downloading, they receive 512kbps, but if one is idle, the other one can go up to 1024kbps.