How do I sync calendars between 2 iPhone 4s's?

Sync them to the same service (e.g. iCloud, Google Calendar, Exchange, etc). They cannot be synced directly to one another.

Two ways, as far as I know...

1. Use the same account name (obviously)

2. Share the calendar:

a. press Calendars button (top left)

b. press blue arrow next to the calendar you want to share

c. Add Person... select the person you want to share the calendar with

d. Set permission level: Allow editing (yes/no)

Not sure if you can share contacts this way. Don't think so...