Simple carbohydrates are going to be your best fuel source (gels, candy, pretzels, chips, various bars, etc.), the key is finding what works well for you while running/hiking strenuously all day. Gels aren't necessary really, they just make getting in those carbohydrates convenient and metered out (typically in 100 calories per gel pack servings), they often contain amino acids which aid in the recovery of muscles. I find tearing the top of a gel off and sucking it down with a little water easier than fumbling around with a ziploc bag and various other carbohydrates while running. Breathing, chewing food, and running can be tricky. I like taking in a small amount of protein in addition to my carbohydrate intake, it takes the edge off of having sugars in my gut all day and helps keep me from feeling like I have a solid mass of jello up front.
We can only digest so many calories an hour, overdoing it on the caloric intake while exercising can wreak havoc on your muscles, due to blood being diverted to aid in stomach digestion. Factor in heat, digestion, high exertion, and you get a tricky situation that requires a little forethought. It all about finding a balance on these long runs, one in which I'm still trying to figure out and have messed up on in the past...but I'm learning.
Of course all this is dependent on the individuals metabolism, level of fitness, and rate of exertion. If you plan to get after it and push it for the duration of the double crossing then eating solid foods may not be as feasible or practical, gels and quickly absorbed carbohydrates will be better. Powerhiking/walking is less intense and lends itself to eating more solid foods without any ill effects usually. If you think you'll be on your feet for 14-16hrs. then gels and candy isn't going to cut it.
As far as getting in electrolytes, like Craig mentioned, salty foods (chips and pretzels) help provide a portion of your electrolyte needs (sodium and potassium), but not the complete electrolyte requirements. S-Caps and Endurolytes are two options that do provide a more complete electrolyte replacement. I've had seriously nasty cramps in the past and have had them subside, barely, after taking in Endurolytes. Hammer HEED, Accelerade, Gatorade, Powerade, etc. are all options for getting in some sodium and carbs, but they're not all created equally, see what works for you.
I'm curious what others will be using as well. Hopefully Art will chime in on this, he has a lot of experience here.
No mezcal and peyote trips? You could do a quadruple crossing and be back home by Sunday.