cuelights, as mentioned are a good method. Should you not have a cue light system (they cost about AU$20 to make each station yourself, thousands if you want a really good system where you use mic lead and daisy chain the stations from the cueing master station...).
String is a fairly good way to cue at a distance when you have no budget. I made a system in our workshop one day where I cut 4 levers for each station - 2 go on the masters end, 2 go on the ops end. They pivot in the middle, and string is tied at one end. I painted one yellow and one green on each end. Tie the string between them using pullies, so that the string is verticle at each end, and the levers are opposite to each other. Standbys, I pulled down the yellow lever, to confirm, they pulled it their end (it is basically a sea-saw, one up, one down on each end). For the go, I pulled the green, then pulled it back in.
(diagram)
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(sorry, it is not the clearest diagram)