One method is to use a variant on Loic Leferme's method.
Once the diver goes down, the moment that he hits his planned bottom arrival time (say 1:20), you start pulling up the bottom weight from the surface, NO MATTER WHAT. The safety freediver then descends the 'moving line' to spot the ascending diver. The diver may still be okay. The safety pull-up is executed on every dive no matter if the diver is okay or not. Since with such a simple setup you never know if the diver is okay, you have to ASSUME THAT HE ISN'T.
Once the diver goes down, the moment that he hits his planned bottom arrival time (say 1:20), you start pulling up the bottom weight from the surface, NO MATTER WHAT. The safety freediver then descends the 'moving line' to spot the ascending diver. The diver may still be okay. The safety pull-up is executed on every dive no matter if the diver is okay or not. Since with such a simple setup you never know if the diver is okay, you have to ASSUME THAT HE ISN'T.