Can 2 people sync to one calendar on iPhone?

How to use Family Sharing with the Calendar app
Launch the Calendar app on your iPhone or iPad once you've enabled and set up Family Sharing already.
Create a new event or tap on an existing one that you'd like to appear on the family calendar.
Tap on Calendar.
Choose the calendar labeled Family.
HTH

THere are 2 ways you can do this.

The first way just involves setting up family sharing, which is quite useful if multiple family members in a household use iPhones as it lets you share content and information, so you won't have to pay multiple times for an app for example. One other thing family sharing does is automatically create a "family" calendar and automatically adds every family member to it.

If you don't want to do this for whatever reason, you can also make a new shared calendar manually. Open the calendar app, tap calendars at the bottom of the screen, then tap add, give it a name and tap done. Then back on the list of calendars tap more info next to the new calendar you just created and you can share it with other people from here.

And you're good. Any new event added to a shared calendar will appear for everyone who can see it. When making a new event you have an option to configure what calendar it's added to. If you want new events to default to being added to the shared calendar, you can change it in the settings app. And finally, if someone else makes a new event in a shared calendar you're a part of, you will get a notification so you shouldn't miss a thing.