Use your ears. Learn to listen for your buddy. If you can hear them, they're close enough. If you can't then you're too far away, or they ran out of air. But your statement really hit the nail on the head.
Actually, they've been referred to as parking brakes because they suck in an emergency. Still, the biggest problem with a car's brake system is the nut loose behind the wheel. Only 10% of all auto accidents have poor equipment as a contributing factor. Far fewer list poor equipment as a primary factor. You can improve the brakes, but they don't mean a thing if your foot is not touching the pedal quickly enough. It's a problem of just throwing money at a skill/awareness problem. I mean, you identified the real problem: you need to work on your buddy skills. Simply adding redundancy allows you to be lazy about that, doesn't it? Will you be buying the same solution for your buddy, aka your wife? If not, why not? I'm not trying to be a negative Nancy here, but trying to help you put things into perspective. It sounds like you need to be a better diver, not a better equipped one. Why buy gear, when training is a better solution?