There's a couple or three things that I can think of for you to try.
It looks like you are sure that the Legato negative rail only needs 260mA (and this might be worth checking using a measured split rail supply which isn't current limited, both rails connected), and are sure the CCS of both Salas Regs supplies 60 or so mA more than this, then the fault should lie in the voltage reg bit of the negative Salas reg. The Legato does however present a capacitive load to the Salas which it doesn't like. I'd suggest powering it through something like 2.2 ohm resistors to eliminate this as a cause of instability once you've got things working on dummy load.
If the available current to the load is restricted by the voltage regulator bit of the Salas it can only mean that it is shunting more current than the voltage setting requires. No LED's lit means either the LED/voltage setting resistor string is disconnected, or one of the active devices there is faulty or wrongly connected. It is best to swap out Q203, Q204, Q205 and Q206 while checking that they are the right way round and confirming that their Idss is good (3-5 mA). I've made the mistake of putting the wrong BC BJT tranny in before now and it is easy to get a jFet 180 degrees out. I'd check/swap just the BJT BC device first to see if that gets the red LED's lit and a b-e voltage across it when under load. Then progress to the others. You should be using an IRF530 for Q206 on the negative reg, yes?
If the negative Salas doesn't power the Legato properly then it will have an effect of how the positive one operates when they are both connected. So unless the negative reg supplies the current the 75 ohm dummy load needs it won't work on the Legato. (Yeah, I know, seems obvious when you say it but I made that error too).
Hope that helps.
John