I often travel with two cars due to the number of people on the trip and have regularly wondered if there is a better way to identify the two cars to each other. Can you buy a small light of a different color or a reflector to put on a car so you can tell it apart from other cars? Would this even be legally allowed or is it considered distracting? What search words should I be using to find the materials for this, or where could I get this? Do you have any other suggestions (besides safe driving techniques, which we already follow)?
Have two passengers, possibly the ones in the “shotgun” seats, communicate via text. Or walkie-talkie. Or cell phone.
Tie a dead cat to the antenna.
Is that one of the 101 uses?
Buy a brightly-colored, distinctive car and make sure to have that one be the lead car.
But first, one must tie a belt to the dead cat, and then tie an onion on its belt, as was the fashion in my day…
Serious answer? I’d make sure that at least one cell phone is each car was using identical versions of the same mapping application. At the outset of each leg of the journey, choose something along the route - a rest stop, a restaurant, a hotel - and agree that’s where to meet up if you become separated.
Walkie-talkie is helpful. Used them going cross-country. Places where there’s no cell coverage you’ve got a backup for communication.
Cat on the antenna is a little Mitt, but to each their own 
Walkie talkies are really, really helpful. Cell phone location sharing apps can be too, but they’re not terrible quick and use up a lot of battery, so everyone needs a charger.
How about having the same antenna ribbons or balls so you (and other drivers) can easily identify all the cars in the convoy?
Lights and reflectors are a no go. Gaudy magnets are okay though. Or car flags. Or antenna toppers, if you want something a bit more subtle.
I’ve done the walkie-talkie thing, and for whatever reason, it just didn’t work as well as you might hope. We ended up calling each other on the phone.
I remember this type of cross-vehicle synchronization being an issue in my youth, but nowadays, everybody has smartphones (well, at least one person per car surely will) with a GPS mapping app available, so all you need is a 12V cigarette port USB charger and charging cable to use it indefinitely in the car. Then each vehicle simply goes to the destination and you meet there. No need to “follow me!” with the fear of losing sight of the guide vehicle in unfamiliar territory.
That’s if you don’t already have a dedicated GPS device in use in the vehicle, or one built into it, in the first place.
If you need to signal an unplanned common detour - “we’re going to stop at Point X to see Y / get food, come join us” - then you make a phone call or send a text message. (Someone other than the driver will look at it, of course, or if driving alone, pull over to the shoulder to read the message, please…)
I like to drive the rear car in a 2 car convoy. I just tell the first car to ignore me and drive any way they like. Even when they get the green and I get the red it all evens out and I have never lost anyone yet. Just make it a tailing game.
How about several sheets of Hi-Viz paper across the bottom of the rear window?
I’m going to against the grain and suggest you do not travel as a group. You’re two separate cars, and you should act as such. Both cars should know where they’re headed, and trying to caravan together ends up causing more problems than it solves.
There’s a reason several agencies forbid groups to caravan together somewhere.
Makes it too easy to set up an ambush.
Tie the two cars together with a long strand of string so that they can’t be too far apart.
Write down on a post-it “Gray Toyota Camry”, so your driver doesn’t forget what the other car looks like. (Caution: Only an example. Do not write “Gray Toyota Camry” unless it actually is one.)
There are quite a few interesting suggestions here (the dead cat one was particularly entertaining. I do have a cat but I think we’d like to keep him alive for the time being). Unfortunately cell phones and GPS only work to a point. We are often traveling to remote hiking locations that have spotty service which can cause difficulties, and also don’t have a specific address to give the GPS. Plus, I have members of the group who still insist on using actual maps to navigate. It’s more fun that way apparently, though I wouldn’t know since I’m usually one of the drivers. I like the walkie-talkie idea and may invest in some, as well as a few antenna toppers - preferably sans dead cat.
Daytime; matching car flags; the kind that clip to the top of a window. If they aren’t the most prevalent flag in the region, others may recognize that you’re ‘together’.
Night time; an illuminated license plate frame on the lead car. The neon ones work extremely well.
In that case, I got nothin’.
Yes on the walkies. We have a car club that we go on drives together with, and this is our method of communication. One for the front and rear cars and, if long enough, toward the middle as well.