In my trip to Peru I found out that my Scarpa Phantom GuideScarpaPhantom Guidehttp://www.scarpa.com/images/products/87411-210/detail.jpgRedefining mountain performance, this boot is suitable for challenging the most technical routes in cold climates, whether ice cragging or in the high mountains. are probably not quite warm enough for multi-day trips at altitude. Well, actually the problem is their lack of warmth, they are plenty warm when dry. However, the problem is that they won't be warm after the first day. And good luck trying to get them to dry when camping on the glacier. Therefore double boots are in order to maintain warmth for days in an a climb.
Fortunately gone are the days of clunky plastics. Best of the current crop of double boots are supposed to be only slightly bulkier than single boots and not much heavier either. Particularly Scarpa Scarpa Phantom 6000ScarpaPhantom 6000Phantom 6000, new model indicate for technical Himalayan mountaineering and extreme use in cold environments. No compromise in the search of innovation in warmth and lightness. looks the part when it comes to volume. According to reviews they offer good ankle lock and climbs as well as the Guides. I've read that they have flimsy inner bootie, that's not very durable. However, this shouldn't be too difficult to fix, as Palau booties can be had for reasonable price. What makes me concerned is the fit though. My Phantom Guides are 45. I'd like HA boots to be slightly larger, but apparently Scarpa makes no half sizes.
Unfortunately pretty much all of the competing models (that is to say La Sportiva BaruntseLa SportivaBaruntseWhen the conditions are cold and high the Baruntse offers a hospitable climate to keep your feet warm and dry. Ideal for use on 6-7000 meter peaks or in harsh winter conditions where you don't want to think about your feet. The thermo-formable inner boot sports the patented speed lacing system and couples with the highly insulated multi-layered PE outerboot for dependable warmth and a PU coating to keep moisture and cold outside and heat inside the boot., La Sportiva SpantikLa SportivaSpantikThe Spantik is perfect for 6-7000 meter peaks or anywhere that you need a toasty warm performance fit. This is a lighter, warmer more compact alternative to the Nuptse. It is step-in crampon compatible and provides excellent technical climbing capabilities., Boreal SiulaBorealSiulaIntense to extremely cold conditions. High mountain and altitude climbing. Long winter alpine ascents., Asolo Base Camp GVAsoloBase Camp GV, Lowa Expedition 6000 RD PLUSLowaExpedition 6000 RD PLUShttp://www.lowa.de/artikel/sehrgross/230065%207220.jpgThis proven mountaineering boot is ideal for tough, western alpine routes with mixed terrain and for expeditions in the 6,000-meter zone. The removable inner boot of quick-drying materials keeps you dry and fits perfectly. and Zamberlan 6000 Denali RRZamberlan6000 Denali RRhttp://www.zamberlan.com/upld/201203/001_6000_Denali_black_red_sml.jpgExternal gaiter made of Cordura® fabric in the upper area and Cordura®/Kevlar® in the lower area to ensure thermal insulation and abrasion resistance - Riri Storm® waterproof and UV ray resistant zip with protective cover - Inner, removable bootee sepcifically developed to grant warmth, lined in mesh for better internal ventilation - Thermal removable footbed with a PE layer for cushioning and an aluminum film for optimum insulation - Carbon fiber + fiberglass midsole ensures maximum rigidity and lightness - Vibram® sole for extended durability and grip - Zamberlan® PCS system for crampons. are either a lot bulkier, quite a bit heaver or both.
Dane of ColdThistle has written extensively on the subject of double boots, both in the form of general articles and reviews of specific models.