Good thinking … but slightly incorrect terminology.
The Apple devices on your AppleID are your Trusted Devices, and those on her AppleID are her Trusted Devices.
Period.
There is no such thing as a Trusted iOS Device on two different AppleIDs.
You CAN add her phone number as a Trusted Number on your AppleID … to selectively receive your 2FA verification codes via voice or SMS … and vice versa.
See details here:
Two-factor authentication for Apple ID - Apple Support
and:
Get a verification code and sign in with two-factor authentication - Apple Support
ADDITIONALLY:
If you setup Apple’s family sharing, it opens up a whole portfolio of useful features … not the least of which is the ability to easily use FindMy to locate each others phones.
See:
Set up Family Sharing - Apple Support