Some answers (as best I can)
I use a Marmot Sawtooth -9c down bag.
For my underquilt I use a DIY full length down quilt with about 4" baffles. It's my 4 season underquilt that I use on my early spring/late fall trips and everything in between.
What temperature is your underquilt rated down to?
When I compare my quilt to some of the others of the same dimensions on the market (eg. Jack R Better Mt. Washington (0F - 10F) or Stormcrow's Winter incubator (10F - 20F), I see that they use a slightly better quality down (higher fill power). I'd say that at -25C my underquilt was max'd out. It was keeping me warm, but just barely. That said, maybe if my top had been warmer, it would have felt better.
With your -9 top quilt and underquilt what temperature do you think you can get down to (comfortably). Assuming you're also wearing some warm base layers, fleeces, wool socks, balaclava as well.
Well, on the first night, at -18C, I slept perfectly. In addition to my down bag and quilt I had some extras. On my head I was wearing a thin balaclava pulled down over my nose and a detachable down filled hood from a down jacket. On my body I was wearing a pair of midweight fleece trackpants, briefs, a thermal top long sleeved shirt and a pair of thick, loose fitting wool hiking socks I reserve for sleeping only. Stuffed into my sleeping bag went a twin sized thin fleece blanket, a wool sweater and a light poly-insulated jacket. My down vest was above my head and I don't think I pulled it in on the first night, although I definitely did for the second. On both nights I had a thick wool sweater as a butt pad. It was slow to warm up, but eventually evened everything out.
In hindsight, I think I could have done a better job on the second night by taking advantage of my winter coat by sliding it over my feet and using it as a footbox cover. Also, on the second night I went to sleep with slightly tighter socks and cold feet. I don't think my feet had a chance to warm up the down very much. I did an experiment and took the socks off of one of my feet, kicked my down vest to the footbox for extra insulation and was amazed that a little while later my foot was actually warmng up a bit. More prep time in front of the fire next time.
At -26, would you recommend having both a sleeping bag and underquilt which are rated to -26? Or is it ok for the top insulation to be not rated that low as long as you have a good underquilt?
Well, given what I said above, I think you can get away with a topquilt that's slightly lighter than your underquilt because you can always supplement it with extra insulation. A good down vest or fleece/wool blanket will go a long way towards boosting it's warmth. My lesson learned was not so much that I need a new winter bag, but that I need a lighter down summer bag that I can use as a liner. I don't plan on doing a lot of deep winter camping, and for me the expense of buying or making a winter topquilt doesn't make sense.
My subsequent follow-up
Yeah, I think I'm a two bagger. For my money, a -30C bag just wouldn't get enough use to warrant the cost. A second bag, on the other hand, could be the light weight summer ultralight bag I've been looking for a reason to get. Having a synthetic outer bag also makes sense from a condensation point of view, but don't forget the frost bib!
Although I could have added a few degrees by adding a quilt cover, there's really not much else you can do to boost it's performance besides strapping a second underquilt on. One thing is absolutely critical is that your underquilt suspension keep it snugly against the bottom of your hammock. ANY gaps will form coldspots. I can't stress enough the importance of this. I went so far as to supplement my standard end to end straps with ones that went up and over my hammock's ridgeline. It's critical to keep that underquilt in contact with the hammock and to make sure the ends are tight enough to not allow much (if any) air in or out.
A frost bib is also important. I mentioned that after the -25C night if felt as if some had sprayed a cup of water on the material around my head and upper chest. It was so damp that I doubt it would have provided much in the way of insulation if I had to stay a third night. Not good! I need to figure out something that'll cover my head (if possible) all the way to my sternum. No I had the zipper on my Hennessy mostly closed, and it was covered in a layer of frost. Perhaps if I had left it wide open, I may have vented better. I'll have to try it out next time.
Finally, just as insurance, I'll be bringing along a couple of those chemical hand warmers, just to stuff in my socks if things get really bad.