I always carry a spare Phoenix RAM when I go diving in case anyone is interested on trying it. I also carry a spare complete single hose regulator as a safe a dive regulator.
A couple of times when the safe a dive regulator was needed (coincidentally both cases was by owners of fairly new Apeks regulators), they both opted to try the Phoenix RAM and they both seem to enjoyed it.
I also loaned the Phoenix once to another friend and he ended up ordering one complete unit that was available on VDH before the weekend was over.
The use of a double hose is simple, but it is definitely different. I have seen people immediately liking it and wanting to use one all the time. I have also seen other that just don't get used to it. There are many reasons why someone may not get used to it, but IMO one of the most common reasons is just newer divers moving out of there comfort zone.
IMO, if you are going to introduce someone to a DH, you need to be patient and explain how to clear the mouthpiece (both by blowing and raising it just a bit) and how the regulator is going to perform. You need to tell them to take longer breaths, not short choppy breaths. And just have them breath for a while in shallow water. At first have them breathe in a vertical position where the regulator diaphragm is low, before they start swimming. And have an octopus handy in case they need it.
The learning process can be as much fun as any part of diving, but there can a bit of a process.
In a comfortable place (and when they are a bit more comfortable), have them play by taking of the tank and placing it in front of them. Then test the breathing performance as they raise and lower the level of the regulator. That is something that everyone should do every once in a while... it is fun.
Note: not to scare you, but in our society (at least here in the US) you do need to be careful about liability. Make certain you are allowing someone to borrow a piece of vintage equipment and that they understand that they are responsible for their own safety. You are not teaching about vintage equipment diving, just offering free advice.
I am not a layer, so I am not guarantee that this will help, it is just my opinion.