Can the Verizon Samsung Galaxy S9+ also run T-Mobile? (This question does not apply to the unlocked version - it applies to the Verizon version specifically)

Is the Verizon Galaxy S9 Plus able to also run T-Mobile using its dual sim feature, or is that not possible?  

The reason for being interested in the Verizon version rather than the unlocked version is for VoLTE and WiFi calling which I've read may not work on Verizon on an unlocked model.
First off, you unfortunately can't get an S9/S9+ in the US that supports dual SIMs. Theoretically, you could import a European model to get that, but don't—compatibility with US carriers (especially CDMA ones like Verizon and Sprint) will be severely limited. 

Second, I'd definitely recommend you purchase an unlocked S9/S9+ from Amazon or Samsung, rather than the Verizon model—not only will you save $90, but you'll also miss out on lots of annoying carrier bloatware, and have the option to switch between carriers like Verizon and T-Mobile anytime you like. It'll still have VoLTE on Verizon and T-Mobile (though not VoWiFi) so you won't be missing much in the way of carrier features either.

Also, T-Mobile is doing a pretty sweet BOGO deal right now—if you're interested in T-Mobile, then that would be a great way to go. Their model is equal in price to the unlocked model as an added benefit.

Hope that helps, and feel free to comment below with follow-ups!