It's totally fine to do a long-sleeved onesie under a short-sleeved one. The other thing I did last winter was layer short-sleeved onesies under long-sleeved outfits for an extra layer of warmth -- and to make sure her belly was covered if her shirt rode up or pants rode down. I probably started layering like this in late October -- but I live in Maine so it might be later depending on where you live. Probably when days are high 40's, low 50's. Before that I did long sleeves and a jacket. My daughter has no tolerance for being too hot so I never did more layers than a 2 and a jacket/coat. At bedtime I sometimes put a long-sleeved side snap shirt or onesie under her jammies. She wore fleece footed jammies and we also use sleep sacks. And her room is kept at 70 degrees all winter. That was good for her. Again, she has a low tolerance for heat and also I was afraid of SIDS and overheating her so I was cautious. She will always complain if she is cold.

I think it all depends on your baby. My son was a super hot baby and hated to be hot. Last winter was his first winter and some days we had to open the car windows for him because he was screaming bloody murder from his car seat because he was so hot. I would only ever put him in clothes and then the snugli for the car seat. And Im pretty sure our winters are close in temp...NY here.

Anyway, if your daughter doesnt mind layering then you should be fine. I never used the long sleeve onsies...but I guess thats a good idea to extend the life of all the short sleeved onsies!! In the fall winter and early spring I do have my son wear a short sleeved onsie...even under his PJ's I dont double onsies...but I guess he does have 2 layers on...undershirt and regular shirt.

my daugther was born end of october 4 wks early so layering was a must with her. I always layered either a short sleeved or long sleeved onesie underneath her clothes. We had one of those bundle car seat covers so I never put a heavy winter jacket on her if she was in her seat. i always put a blanket over her carrier when we were outside to protect her face from the cold air when we were going to and from the car.

I agree that when you start depends on where you live. I live in Southern California and what we consider cold is not what other people consider cold! LOL
And I agree that it depends on the baby. I think my oldest was a hot baby, but my second was a cold baby, and my third seems to be somewhere in the middle!
I love to keep onsies under cute outfits because it kept their tummy covered.
During cold weather we don't keep our house too warm because I can't stand the hot air from the heater, but my older kids were always fine in regular jammies at night or onsies and socks- but we co-slept. It seems like we would have them in footed jammies with a onsie underneather before bed, but just in the onsie when we got inot bed.

Something that I think is handy are these little covers I keep in my car just in case the evening gets cold. I do not know what they would be called... they are a one piece body suit & look like footed jammies without the reinforced feet. They have hoods and the sleeves have extra fabric you can fold over to cover the fingers.
When we have my son (5 mos) out in the evening and it gets chilly we just put that over his outfit. The ones I have are from my oldest son who is now 12 and have matching sleep sack, so there are times we are out at a soccer game and I use the body suit thing and the sleep sack and a blanket.
I always remember the rule that one should dress the baby like yourself plus maybe one layer. Then I check fingers and toes and cheeks often! A hat can do a lot more than a layer at times by helping baby keep in more of their own body heat.

Oh, I don't know how people with truely cold weather do this because like I said I am in southern California where it might get a little nippy (we have some crisp 35 degree mornings on occassion but nothing like the freezing temps other places have...) but when you bundle your baby you have to take into account how you will buckle the baby into the car seat!