sheperdsflock the reason why your toes start freezing is because your feet contaminate the insulation part of your cross country shoes with sweat. At -20F forget about cycling shoes because they all use the same technique which is pretty much useless at those temps.

Below 10F it is better to use the vapor barrier technique rather than add insulation layer after insulation layer which all will be contaminated with sweat one after another.
The army already knew that at early 70's. The bunny boots for instance can sustain -65F (-53.8C).

To sum up, the answer to your problem is to add a plastic bag over your socks to avoid contaminating the thinsulate insulation of your cross country boots and if it's not enough insert your cross country boots inside some 30$ 7mm neoprene boots
Total cost : 30$ + 1$ = 31$