How about this:
Completely rebuild (replace parts) every two years - alternating posts. The complete rebuild always goes on your left post and you take the former left post reg and take it apart, clean, lube, and inspect it. If all looks good, which it will barring any neglect, transfer it to your right post. Both regs get annual cleaning and inspections, but you are only replacing seats and o-rings every other year on each reg. That way, both regs are in pretty much new state all of the time and you are only doing a complete overhaul on one reg each year.
I am assuming you are runnning your long hose primary off the right post. If so, then you are always using the older reg as your primary and, even with a fresh cleaning and inspection, you are living with a high pressure seat with a season's worth of pressurization, even if not many cycles and a second stage with a well developed seating groove that in most cases costs you slightly higher inhalation resistance.
Plus it's a major mistake to ever dissassemble a reg and then re-install old seats as unless things line up perfectly and with zero relative rotation if the parts involved in holding the seat and orifices are less than 100% concentric, you can end up with overlapping seating grooves and eventual leaks. Seats are cheap, if the reg gets dissassembled, replace them. In some cases you can flip them over and use the other side, in which case switching seats is essentially free.
If you use the switch the regs on the posts apprioach, I'd suggest doing the opposite. Put the freshly rebuilt reg on the right post where it will get maximum use and break in quickly. Then after 6 months, 1/2 season, etc, switch it to the left post and put the newly rebuilt backup reg on the right post as the primary.
The same thing works if you rebuild both at the same time - swap them mid season with a new one on the right post followed by the less used backup being rotated to the right post mid season. It will even out any wear occurring due to number of cycles. Some regs when used only as a primary reg can have a higher probability of a problem after 100 dives or about 50 hours of primary use underwater. So if you do more than 100 dives per season, swapping them mid season is a good idea. Otherwise, there is no significant advantage to it.
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I also think that using your primary or backup regs to the point of a problem occurring is pretty stupid if you are doing overhead dives. In my opinion, if you needed to swap regs during the dive, you screwed up. The backup reg should be totally redundant and only along for the ride in the event your buddy screws up and goes OOA (likely the last dive with him or her until he or she gets their stuff together) or unless you have a tank related gas loss of some sort (but those should also be very rare if you change burst discs at least at every hydro and change tank neck o-rings at each VIP.) Delaying service until you need it is liek planning to use your gas reserve during a dive - it is a practice that will eventually bite you.
Technical diving is expensive, but skimping on reg service is not the place to save money. You can reduce service costs by learning to do it yourself - which is a good idea for a technical diver you usually end up owning at least 4 or 5 regs and also as knowing how they work will help you prevent and resolve problems if they occur.
You can also save money and extend the "safe" interval between services with proper maintainence, storage and thorough pre-dive inspection, but you need to know what warning signs to look for and also be willing to cancel a dive if you find a problem rather than dive with it.
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Personally, my thoughts are that it makes sense to get them both done at the same time, or one right after the other. If you have an opportunity to do a few quarry/training dives right after the annual service, get them both done at once. If not, get one rebuilt and then 3 to 5 dives later get the other one rebuilt.
