Spin the World Around was a concept based around empowerment through solo travel. I've extolled the virtues of traveling alone over traveling in a group, but two is often the perfect number of people for travel. Traveling with a friend or partner can make a trip more economically and socially viable than traveling alone or traveling in a larger group. Here are specific reasons to travel as a pair, and types of trips which are best suited for bringing along a travel partner instead of going at it alone...
Based on Double Occupancy
Most all-inclusive resorts, group trips, retreats, cruises, and other organized excursions that require stays in a hotel room or sharing equipment quote their prices based on the assumption that the booking will include two people. It ends up being negligibly more of a cost to support two people than one, especially when sharing a room, meals, and side excursions together. This means that if you try to book with a single person, they will charge a premium, and as an individual you will pay more. Therefore, any travel that involves an entire room (as opposed to a couch in the corner of someone's living room) will often have a cost benefit if you travel in pairs.
The cost of gas and car maintenance is the same no matter how many people the car is carrying. Therefore, bringing another person along for the ride can cut your expenses in half. It also helps to be able to share driving responsibilities, which will not only get you there faster, but could possibly lead to more time spent on the road moving forward instead of resting at more hotels along the way. Not to mention, having a witness to all the glory you will experience on a road trip can help make the trip more impactful as you spark each other's memory about all the awesome things you saw!
Who's Got Your Back?
Of course, having another person around as you travel is a great safety valve for when things get weird. Your travel partner can be a good 'rip cord' if you ever need to get out of a situation, as well as a "another set of eyes" to help you judge a situation before it gets out of hand. That's not to say that you should have to babysit each other, but it's good to have each other's backs.
Minimal Social Effort Required
Having a travel partner with whom you get along means that you don't have to spend an inordinate amount of time going out of your way trying to be social. When I traveled alone around the world, there were stretches of loneliness that were only filled by either meditating alone or putting forth the effort to meet new people in order to have a social interaction. When you have someone to fall back on during your travel, this no longer becomes important. Of course, it can be a double-edged sword if you begin to isolate yourself from the culture which surrounds you, but in reality, if you want to still meet locals, it's not too much harder to do so as a pair than as an individual. Once you start to get a larger group, the island of isolation can grow bigger, but with only two people, you can have the best of both worlds if you wish.
Solo Trip Within
When traveling in pairs, there's no rule saying you have to be together the entire time. Once you reach a destination such as a city or resort where you have a home base such as a hotel room or cruise cabin, there can be plenty of opportunity for the two of you to spend a little time on your own exploring the things that you might deem important which your travel partner does not. For example, I'm not a huge shopping fan, and my girlfriend is not scuba certified, so an afternoon apart lets us try different things and then have a ready-made conversation about our day when we come together afterward.
Travel Partner or Couples Travel?
It's always good to be clear up front about expectations when traveling as a pair. It's inevitable that there will be some deep bonds formed as you create lifelong memories together. The only question is whether those memories will be of fondness or awkwardness. This can especially be uncomfortable if one person is paying for the entire trip. Does this include attached strings? Is it really a business trip? It can be important to know up front the nature of the relationship, lest you end up in a hotel room with one bed in the middle of some foreign place with a dude coming on to you who painted a very different picture in his head about how the trip would unfold. If you're both on the fence, it could be an exciting catalyst to see how your relationship evolves, but if you have a clear concept of a platonic relationship with your travel partner, it's best to lay the groundwork before you leave home so that nobody's feelings are hurt on either side of the equation with nowhere to go.