What's the best way to sync iCloud keychains between myself and my wife?

When two Macs are signed in between same Apple ID and password on the same network , and iCloud keychain is set up on both the passwords will be shared Set up iCloud Keychain - Apple Support

You can export some keychain items (for example, certificates and keys) from one computer and then import them into Keychain Access on another.

Note: You can’t export passwords from Keychain Access.

According to this article Import and export keychain items using Keychain Access on Mac - Apple Support

You can't combine keychains in a Mac .

According to this article Copy keychains to another Mac - Apple Support

If you migrate your data to a different Mac using Setup Assistant, your keychains are automatically transferred to the new computer.

If you didn’t use Setup Assistant, the best way to copy your keychains to a new computer is to export and then import them using Keychain Access. See Import and export keychain items.

Note: You can’t copy passwords stored in your Local Items or iCloud Keychain. To transfer these keychain items to another computer, set up iCloud Keychain on the other computer using your iCloud user name (normally your Apple ID) and password. 

You can manually copy keychains other than Local Items or iCloud Keychains to another Mac using the steps below.

1. On your Mac, locate the keychain files you want to transfer.
2. Keychains are usually located in the Keychains folder in the Library folder in your home folder. To locate these files, press and hold the Option key and choose Go > Library, then open the Keychains folder.
3. Select the keychains you want, then press and hold the Option key and drag the selected keychains into another folder to copy them to the folder.
4. Keychain files typically end with .keychain-db. Don’t select and copy encrypted folders with names that appear as a series of numbers.
5. Transfer the folder containing the keychains to the new computer.
6. Important: Transfer the keychains in a safe manner so no unauthorised person can access them. For example, use AirDrop or a USB flash drive to copy the files.
7. Change the name of the transferred keychains on your new computer so they don’t become confused with any of the existing keychains. 
8. This is especially important if you are copying a standard keychain, such as the login keychain.
9. Open Keychain Access, located in the Utilities folder in the Applications folder.
10. Add each copied keychain one by one by choosing File > Add Keychain, selecting your keychain, then clicking Add.