This is slightly off topic, but I'm curious ... why are alternate seconds (even within the same model regulator) so much more inexpensive? What I mean is ... it does make sense that you'd want your primary and secondary to be the same, especially if you're in a primary donate situation.* Using Apeks as an example, their website lists the MSRP for a primary XTX50 at $599 while the MSRP of a secondary (yellow, otherwise identical from what I can tell) at $270. The same is true with the AquaLung Legend Glacia: $855 for primary and $275 for secondary.
I get the desire for a less expensive secondary since, for all intents and purposes, it seems as though the secondary will be used less frequently by comparison.
*The implications of this statement are that, in a primary donate situation, you want to make sure you're secondary regulator (the reg you're now breathing off) is "as good" as the one you just donated but in a secondary donate situation, it's okay for that poor schmuck out of air can breathe from a sub-par reg. That wasn't my intention, but its a bit of an interesting thought.