There are many reasons to have a second photographer to assist in capturing an event. The most obvious is that it's impossible to be in 2 places at once and having another pair of eyes and hands can be invaluable. Also, with high profile and unrepeatable events like weddings, it's another built in safety layer of backup in case the absolute worst should happen. A second should be more than just a wedding photography assistant. I don't bring people with me just to hold coffee and run errands. Here are the things I look for when I'm bringing a 2nd.
1. Reliability / Communication
Without this, pretty much everything else is worthless. You can have the most awesome portfolio, the greatest gear, the willingness to learn but it doesn't matter. When it comes to someone's wedding or event, there's no room for excuses on why you can't. I deliver every time and I make it a point to make sure that anyone that is tied to me can do the same.
Really, it's ok to smile. The best part about this job is being able to have fun and relate to people of all types. A second should be able to have fun and mesh with both me and the guests.
3. Sample Images / Portfolio
Everyone has their own style. When shooting for me, I have 2nd's provide their RAW files without any sort of editing. What I look at in their profile is if they have a basic composition style that can mesh with mine. It doesn't matter if their photos look warm, cool, moody, airy or what. When it comes to event shooting, it's completely different from portrait shoots. You don't have time in most cases to pose people. You need to be able to adjust for varying light, random shifts in attention and pretty much expect the unexpected.
4. Gear (Within Reason)
Gear is a touchy one. While i'm a firm believer you don't need over the top expensive gear, you do need the correct tools for the job. Below are the minimum of what I look for.
-Camera body with dual card slots + 2 cards
-At least one lens with an aperture f2.8 or below
-An extra battery
Short list right? It's because a lot of answers are built into that without having to play 20 questions and micro manage. Here's a quick breakdown
"Camera body with dual card slots + 2 cards"
No manufacturer is specified because it really doesn't matter. These days all the major brands make good cameras and it comes down to personal preference. It doesn't matter what kind of file they output because I can edit any format. Making sure it has dual card slots meant that it there's less of a risk of image loss as well as it's going to allow only "semi pro" cameras from each manufactuere. (Yes i know the new high end Nikon Z mirrorless and Canon mirrorless have single card slots. Just because it's expensive doesn't mean I would risk losing images as a whole to a single card failure)
"At least one lens with an aperture f2.8 or below"
Having a lens with a low f stop means that they can shoot lower light without going to a higher iso and have an easier time creating depth of field separation. It also means that they're either using a higher quality zoom or prime lenses which are both fine in my book.
"An extra battery"
That one is kinda self explanatory but definitely check.
It never hurts to have references. Generally, if they've worked for another wedding photographer you trust and get good reviews, it's a safe bet that they'll be ok for at least some of the check list.
Overall with these things, the bases are covered and you're free to do what you do best. Deliver amazing images.