Robert,
On the real semaphore railway, home and distant signals on the same post are "slotted" in the balance lever arrangement so that the distant arm cannot come off unless the stop is also off. However, provided the box in advance has given line clear and cleared all his stop signals (which implies line clear from the box next in advance), there is nothing to stop the distant lever being pulled off too. Once the box in rear pulls off the associated stop signal, both arms will come off simultaneously. I regularly used to see that at my local station of Castleford.
On Retford (the model), there is no mechanical slotting but it is done electrically. In the up direction, South box can pull off the distant arm on Queen's boards so long as the appropriate platform starter is also off, even though North box has not yet cleared the Queen's board starter. Once this is pulled, the distant arm will also rise. To the best of my recollection, all the distants on Retford are electrically slotted in this way. There is thus no reason for each box operator to watch what the other is doing. The train drivers are the ones that need to do that!
To reply to another point, purely in signalling terms, platforms and signal locations have nothing to do with each other - a signal does not need to be present to admit a train to a platform or another one for it to leave. It's just that most station signalling was laid out that way due to the presence of other features such as level crossings, crossovers, siding connections etc.