With a light aircraft, sharing the costs among two or more owners is common. If you own a small business, using a light aircraft may be ideal and time-efficient. Sharing the costs with another owner can bring down the threshold of cost.
Successful shared ownership requires consideration of the Three C's: Compatibility, Compromise and Contracts.
There needs to be a degree of compatibility between the owners. The type of aircraft must be suitable to the owners' missions. A Cessna 206 and a Mooney Ovation have different strengths. So do a PC-12 and a Piper Meridian. If you are becoming a second (or third) owner, make sure the aircraft will be effective at what you need to do: big load hauler or speedy cross-country machine. Just as important as the mission is NOT having similar flying schedules. If both owners need to use the aircraft every Monday thru Wednesday, sharing cannot work. Ideally owner #1 is a weekend flier and owner #2 is a business-weekday flier. Discuss in advance what your expectations are and how you will schedule the use of the aircraft.
Even if the owners have compatible aircraft requirements and complimentary travel schedules, there needs to be compromise. There will be times when you need to allow for flexibility in the schedules. Visiting family for Thanksgiving? Maybe this year you get the plane and your partner gets it for next year. Other compromises may involve the maintenance and upgrades. If you are a heavy-IRF flier and your partner isn't, then your requirements for upgrading the avionics will differ. If you fly for your business and take passengers, the interior standard you have may well exceed the pleasure-pilot.
Lastly, there needs to be a contract outlining the sharing of the costs and the responsibilities of each owner, and perhaps most important: a way to end the shared ownership. Will you split the fixed costs like hangar and insurance along ownership share? Will you set up a reserve account to pay in advance for the maintenance? What about unscheduled maintenance, how will you split the costs? The engine may cost $38,000 for an overhaul, or cost well over that if you want to do an exchange. How soon do you want that engine back? What do you do if one owner wants out (or cannot afford to stay in)? What if you ant to take on an additional partner? This should be in writing to keep the relationship as amicable as possible.
Before entering into a shared ownership, sit down and really look at the costs. The hourly rental at the local FBO may seem high until you figure out the cost of the initial investment and fixed costs. Sharing the ownership of an aircraft can lower to cost to access ownership, but everyone involved needs to work together to maximize the utility of the aircraft.