Switching phones between family members

Unless your wife is planning on using a local sim card from whatever country she is travelling to, this should not be necessary, as her carrier can provide a roaming package for her trip.

If she wants to use a local sim card, then, and assuming you use the same carrier at home as her:

1. Back up the phones to iCloud or iTunes: How to back up your iPhone, iPad, and iPod touch - Apple Support
2. Wipe the iPhones clean: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support
3. Take the sim card out of each iPhone, and swap them: Remove or switch the SIM card in your iPhone or iPad - Apple Support
4. Set up each iPhone from the other's backup made previously: Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Back up both phones, then follow the instructions here on each phone. Remove the SIM cards and swap them, then set them each up with the appropriate iCloud account and restore the backups you made. (restore the 7+ backup to the 8+ and vice versa)