which means two regulators - either manifolded doubles, or if diving a single tank, an "H" valve with two independent first stages.
The backup is bungeed around your neck - this is important to consider when buying regulators because - that's YOUR emergency air source.
I'm not DIR. I don't profess to be. But I dive identical ATX-50's.
In terms of whether they need to be identical, I believe (someone may feel free to correct me) that such distinction is moot. If an ATX 40 or AT20 or whatever is the right tool for the job, go right ahead.
I mean, let's face it, Apeks makes quality regulators. Just don't try to carry around one of those crappy POS low-profile plastic octo's, or even worse - an Air2.