Every single Verizon iPhone X is unlocked for GSM use and must be unlocked at point of purchase. For security when you order a Verizon iPhone X is ships with a verification step to easily activate the Verizon iPhone X on the Verizon line and phone number you ordered it on.
Just put in your active T-Mobile SIM before you turn on the new iPhone X. Once you turn on the iPhone X it will prompt for the information used to order the iPhone X. Just enter the information and keep hitting "Next" and it will bypass the activation check. Do Not call Verizon unless you want your wife to need to use the new iPhone X SIM in her phone.
Dave
Apparently doing this on iPhone X sold on BestBuy will result in the device being locked to T-Mobile, according to the experience of other users in that thread. Just be careful.