My major concern comes from the fact that some costs are purely calendar-driven, others are purely hour-driven, and many others are somewhere in between. For example, if you hit a dry spell for a few years where only one guy flies the plane but he doesn't fly it much (say 15 hours a year compared to the other guy's 0), the engine will likely need an overhaul before it reaches TBO, but the guy who was at least making an effort to keep it running might unfairly have to pay 100% of the overhaul cost just because his partner didn't fly at all and their agreement divides maintenance based solely on hours of use.
What did you end up doing ?
I am in two co-ownership setups on aircraft. One is a Corp with 5 shares, the other an LLC with currently 3 ownership interests. Both function the same. A monthly charge that covers hangar, basic annual fee, insurance, databases. For every hour flown, money goes into a separate account to cover overhaul reserve and and hourly maintenance (PM and things that break). Once a year we meet, go over the budget vs. expenses, decide on upgrades etc. It worked for 15 years before I joined and has worked the 6 years since. When we did an overhaul and came up short vs. the reserve account the assessments were paid in equal shares, not prorated by the hours.