You can do this by signing into the same iCloud account ID on the two phones in Settings>iCloud. However, a word of caution: when you share an iCloud account, any data you sync with the account (such as contacts and calendars) will be merged and the merged data will appear on all devices. You may, for example, end up with each other's contacts on your phones. You will also be sharing the same free 5GB of free iCloud storage for all your iCloud backups rather than have separate accounts, each with 5GB of storage space. Because if this, it's usually advisable for each person to sync their devices with their own, separate iCloud account.