what wmsontra is saying holds water, heat is what kills filaments, actually spikes in changing temperature kills them quicker than anything else.
But extra heat is not going to make them last longer that is for sure.
If you want your bulbs to last longer, put a large cap across them, the cap takes the initial burst of current and the bulb comes up slower which will prolong its life, the fact is, it ain't at all worth it for a 10$ bulb.
My concern with the switch in the middle would be arcing the contacts and melting them away, also voltage drop across higher resistance contacts.
If you want to run both filiments go for it, but wire it that way with a toggle switch not some magical sweet spot in the middle, as soon as you go over a bump I bet it pops out of that spot.
But hey go for it.
If you want more light, just get an offroad bulb, 80 / 100W but then you got the folks that say it beats up the charging system, but here you are running two filiments at once. A 55W and a 60W that is 110W but I would be willing to bet you are not getting full power out of the bulb in this 'center' spot cause of the losses / high resistance contact.
I don't understand why some people bother with doing weird stuff, if you want more light, get a light that makes more light
2000 Vulcan Drifter 800
Roadhouse classic 2 > 1 exhaust
Okie Smokie intake config
52K miles or so, always going up
Painted flat black winter '12