Hmmm . . .I've definitely said to myself more than once when I wanted a new toy "I can keep the old one around as a spare if so and so comes to visit and wants to go camping" as a justification for buying a different version of something I already have.
I've been doing my best lately to shed 'spare' gear, for space and financial reasons, but also to stifle my inner urge to be a pack rat. I'm no longer going to keep two things that duplicate each other, like my TT Rainbow and my Duomid, even if I really like both of them, but I will own a Duomid and a poncho tarp, as they're different enough, so in that vein, I think I could still probably put together a 'make-do' kit for one other person for a three-season trip, just based on having stuff for different seasosn and conditions.
Let me see. . .
I have two backpacks, one for summer, and a bigger, heavier one for winter. I could sleep under my poncho tarp, he could borrow my Duomid(I don't like sharing tent space. I need my alone time) But we'd need to camp someplace where we could find at least one long stick, as I only have one set up trekking poles.
Quilts would be tougher, but right now I have a 30 degree quilt and a 40 degree quilt that could be boosted by wearing insulated clothes I own(not the lightest option, but do-able) But I intend to trade out the 40 for a much warmer quilt for winter that'd be too warm for summer. But I really should have a synthetic summer weight quilt too, right:)? I have a NeoAir short, and a couple three ccf pads for winter camping, so we're set there.
General hiking and workout clothes, I have plenty of. My poncho tarp is my rain jacket, so he can use my winter shell. I have a pair of golite reeds for summer and a sturdier pair of rain pants for winter. He could use my MB UL Down Inner parka, I could use my synthetic parka. Hat and gloves, plenty of those(never know when you're gonna lose one, so keep spares) Everbody owns a pair of joggers, right? So he'll have his own shoes.
We can share a stove, (or I have an alcohol stove, a white gas stove, and a cannister stove)and I can use my trapper's mug while he borrows my 1.3L winter pot. Plastic utensils are fine. Water purification can be shared, water bottles can be bought at Walgreens cheaply.
I only have one compass, so he'd better stick close. I actually only have one headlamp, so we'd have to plan around full moons.
I think I've covered all the bases, right?