My experience was that 1 and 2 person tents were about the same temp in cold weather, as they are very well ventilated (lots of mesh area under the fly cover) to not accumulate condensation, if not a winter tent. If your body is heating up the tent, your sleeping bag is not doing its job. Since a 2 person tent was only marginally heavier than a 1 person (poles about the same, only a bit less fabric) but half the space, I went 2 person, it gives me space to keep gear out of the rain if needed. That might be different if I backpacked now, because I bring hiking poles and they have non-freestanding tents that can use those instead of tent poles, so the tent might be a lot less weight, but this doesn't apply to bike camping. These days I ride a folder so would definitely bring a 2-person tent, fold the bike, put inside overnight to prevent theft.

When I toured with a long-wheelbase recumbent with trailer (tons of room in a giant rubbermaid box, I didn't even need to compress and stuff the down sleeping bag), I had two rectangular down bags that I used to zip together when camping dual, and if colder weather, I would either bring both of those and put one inside the other for warmth if needed (very versatile, wide temp comfort), or bring one bag and a fleece bag that could go inside the down for warmth and as a liner. If not, I always brought a silk liner to keep the bag clean. When camping on a beach with large flat rocks near a major city (cold, no mosquitos so no tent), I used the black fleece bag on the outside of the down bag, both to protect the lightweight nylon fabric, and as night camouflage to keep from being spotted (black, and non-reflective).