I use a very lightweight Exped drysack rucksack thing, like, but lighter than, the Alpkit ones. Belay jacket, gloves, snack, mini headtorch and perhaps a drink. Weighs a fraction of dragging the full sack up.
Either the above if it's a short route (like those in Idwal) and the descent is quick and easy back to your pack,
Clip a water bottle and belay jacket (if you can) to your harness and carry the head torch in a pocket or on the helmet.
Like you suggested, one rucksack beteen you both and the second brings it up
Or, carry a small light pack like the Arcteryx cierzo 25 in your main pack and use that. It's small, light and packs into its own lid pocket. Pull it out at the base of the climb, stuff what ever you need into it, and off you go
One word of advice though. If you're climbing at Clogwyn Du in North Wales and leave your pack at the base of the climb, be warned that ravens may raid your sack for all your edible goodies. This happened to me last year, little buggers are surprisingly intelligent. So make sure you leave it out of sight but remember where! Stops little gloved hands from having a rummage too.
Torch & food in the pocket, guidebook on the second's harness, map/compass between the layers somewhere, water I can do without during the climb (before/after ok), belay jacket I can do without if need be (maybe another mid-layer to compensate). In Scotland I've generally always either taken both sacks or left both sacks. When I've left them it's been because they will get in the way (even for the second).
Car keys, compass and phone in pocket.
Pocketful of munchies.
Small headtorch in pocket, or bigger headtorch on helmet - not so much for finding way off, but for finding the sacks if plan A is not quite as well-founded as originally thought. I always try and leave sacks by something obvious that can be found in darkness. For example, 'the southernmost point of a patch of rocks' is easier to find than 'the obvious boulder that looks like your Mother in Law'.
Guidebook if necessary in one gaiter; map in other gaiter.
Goggles if necessary for wild weather on top, on helmet.
Belay jacket in stuff sack that clips to back of harness (or one of those cheap promo gym shoe/clothes bags - a stuff sac with cords that can be worn as a rucksack - if this option then torch and goggles go in here too)
Spare inner gloves and knife in back pocket.
Spare tat in thigh pocket.
Blair Fyffe and i left our sacks in the gully near what was to become "Rhyme of the ancient Mariner" on indicator wall. It snowed heavily whilst we were on the route and thus the decent down the gully to get them was too dangerious.
It then snowed for about a week. Blair's car keys were in his sack.
We went up a couple of times a week that winter and probed and dug for them and eventually found them a couple of months later about 4m down in the snow pack. On the upside we cleared a platform about 4m by 3m that everyone appreciated as a great gearing up spot.
Unless it's cragging with an easy decent or abseil, I'm reluctant to leave my sack, in case I can't easily retrieve it.
Like others I have learned from mishaps in the past, trying to get back to my sack have involved long and knackering detours when all I wanted to do was get back to the car!
Most decently packed climbing sacks will be almost empty by the time you gear up.
Once the hip belt and lid are folded away and the sack is compressed, you will hardly notice it.
On Scottish, Alpine routes with ab stations I use a BD Bullet 16 litre bag to keep my spare gloves, belay jacket, 1st Aid Kit, headtorch, phone, food and water in.
The bag is tough and packs small and weighs very little. I also save weight in my approach sack by leaving the main sack lid behind and using the Bullet for carrying bit and bobs on the walk in.
Whatever you do, make sure you have your head torch, phone and fist aid kit as well as spare gloves and a belay jacket with you!!!
I found a rucsac on Aonach Beag one Spring day, slowly melting out of the snowpack... Luckily there was still enough battery power in the phone (and no security) to contact its owner. And some chocolate for me to steal
Very similar story to mine, except for the packs (and car keys) were at the bottom of monolith groove in Arrochar.
We had to hitch hike back to Glasgow and my gran in Bognor got a call from Strathclyde polis about "her" car being abandoned on the rest and be thankful.
Fortunately for us, it rained through the night (which had made the descent gully too dangerous to descend and led to my mates minor hypothermia needing a swift exit off the hill before we could slog back up to the sacks after coming down the long way) so got the bus back the next morning, hiked up got the sacks, got the car. Back in Glasvegas in time for brunch.