Can the Verizon Samsung Galaxy S9+ also run T-Mobile?
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!