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When you have a twinset, you have to give some thought to what goes on which first stage, and how it gets routed.
There are two main schools of thought:
1. If a hose is connected on the left, put it on the left first stage
2. If a hose is connected on the left, put it on the right first stage
The advantage of the first is that it makes for simple routing.
The main reason people seem to give for the second is that it results in most hoses being routed behind the neck. This allows you to hear pinhole leaks in the hoses.
Personally, I use a bit of a mix. When I sat down and tried to work out which was the best way to do it, my basic thinking was:
First and foremost, you need a primary DV. For reasons explained elsewhere, I have a long-hosed primary on a Hog loop. This would be awkward on the left post as when donated it would pull my head forward. So it went onto the right post.
For a twinset to provide redundancy, you need one demand valve (DV) on each first stage. Therefore the short-hosed secondary went on the left post. Since the DV hose must come from the right, it must be routed across from the left to my right.
That's the DVs sorted. Next, I have two direct feeds to think about.
The wing can be inflated by direct feed or orally. The drysuit can only be inflated by direct feed. Therefore you want your drysuit direct feed to be on the reg least likely to go wrong.
The right post is the one used throughout the dive to breathe from. It's also the one an OOA buddy will take. It is the most likely to suffer a problem, therefore the suit direct feed should go on the left post.
The wing inflator can function as a source of gas if the need should arise. The only real reason for using it, however, is where you have donated your primary DV but the secondary isn't working. If the secondary is not functioning, then the wing direct feed will only function as a source of breathing gas if it is on the right post - a post you know is working as you were breathing off it seconds ago.
Therefore the wing is on my right post and the drysuit on the left. The wing is routed behind my neck, the drysuit feed goes under my left arm.
That just leaves the SPG. Left or right?
Again, we must ask ourselves "Which is the most likely reg to fail?". If we shut off its post, the SPG becomes useless. Therefore the SPG should go on the 1st stage most unlikely to be shut down. The left.
Some people do use two SPGs, but I don't. If the SPG stops working, then you call the dive and head straight up - you either have enough gas for this ascent or you don't, the SPG won't add any benefit. Similarly, if you do shut off a post, then that signals the end of the dive, and again it is of no use to know how much gas you have - it's either there or it isn't. Putting the SPG on the most likely-to-survive post is worthwhile, taking two isn't.
And lastly, thank you to Mark Emery on the DIS-UK list, who pointed out that by routing the backup DV hose UNDER my wing's corrugated hose, it would hold the wing properly upright. - that was something that had been bugging me.