Right now, it's about a 3-4 hour trip by car. If you have business in the other, which happens all the time, it either means you have a very long day driving back or have to spend money on a hotel. They are really too close to fly economically and with security wait times, probably better off driving.
If it gets built and works, this would be a great base to expand off of. Denver is a 9-hour drive west on I-70 from Kansas CIty. Expanding to Dallas via Oklahoma City and Witchita wouldn't be much of a stretch either using the I-35 easments. Chicago and/or Indianapolis could be targets to expand to from St. Louis. Denver, Dallas, and Chicago are in the 3 of the 4 other US routes being considered.
Kansas City and St. Louis are connected via I-70 which has a large easement and not a lot of development nearby outside of both metros with the exception of Colombia. So it could be built above ground without many obstacles.