We're talking Toronto here, right? Not Winnipeg, Manitoba! Simple solution is just add another thin layer over your bibs, though if I were riding my bike all day long I wouldn't want to be wearing bibs. At some point I'm going to have to use the bathroom, and with layers upon layers over the bibs...well, you get the picture.
For me, at -15C, which is might be a handful of days in January and February, and usually not all day long, I'd wear a pair of cycling tights with a pair of loose fitting xc skiing/running tights over it. Or possibly a pair of base layer tights and then cycling tights, or in your case, bib tights, over them. That should serve you well enough for those handful of days.