I've seen whippets while skiing and am thinking of taking one (or two?) on a 4 day tour in glacier terrain, but am confused as to their purpose. Do they replace ice axes or are used in conjunction with them?
Whippet is an attempt to add some ice axe capabilities to a ski pole.
- one whippet
- one whippet and one ski pole
- one ice axe and two ski poles
- one ice axe
I don't think two whippets makes sense.
Personally if I think I might need self arrest then I want an ice axe.
If I am going to ski down something steep enough that I am worried about stopping if I fall then I want two ski poles to minimize my chance of falling (and probe for snow conditions).
A whippet when you have conditions you feel you can stop (arrest) with a whippet. And you don't think you could stop without one.
Disclaimer: I don't use whippets and have never even hold one in my own hands.
From the build of a whippet and a question about whippets on AAI (provided by Ben Crowell) it has a very narrow use case: Difficult skiing terrain, where holding an ice-axe significantly decreases stability and thus increases chances to fall, and well known terrain and conditions.
Well know terrain and conditions is very important, as whippets can only help to quickly arrest when you slip, but they can't fulfill other requirements of an ice-axe: Building a T-anchor, plunging and ascending on ice (at least it is only of limited help there). If you are sure that these factors (and probably more that I forgot) are not an issue on your specific tour, you can leave behind the ice-axe.
So for your use case of a glacier tour over several days it is certainly not a replacement, as you need the ability to build a t-anchor to get someone out of a crevasse. And unless you plan to ski very difficult and hard packed sections, it is probably not worth taking them as an addition.
And as to whether to take one or two whippets: If you take whippets, you probably want two. Otherwise you had to swap them whenever you make a turn in descent.