I live in an apartment as well and also have to walk up 2 flights of stairs.
It's not difficult I used to carry DS in my arms and If I had shopping I would carry DS up in the baby carrier and do 2 or 3 trips if I had to.
Now that he's older (19 months and walking, I STILL carry him coz he runs off otherwise.
I often carry 4/5 bags of heavy shopping, DS on my hip, handbag over shoulder as well as DS's esky / backpack / etc
IMO a bugaboo carry cot is worse than carrying bub on his own. I know because I had a bugaboo. It's too heavy and awkward.
I would say a capsule MIGHT be lighter although I've never used one. I also used to carry DS without the carrier when he was tiny.
I go out a lot during the day, at the moment twice a day and I always carry DS. U just get used to it.