I "think", that there are two valves in order to be able to deflate the kite faster.
And by naming one "inflate" and the other "deflate" - it will be easier for the user, as "no choices" is the most simple
Maybe some has slightly different sizes, so one can use different pump nozzles - would be great.
I once tried a one-way inflate valve.
And that was IMO useless, as when you reach a high pressure level, it will require a huge extra load/pressure, to be able to pump air inside (open the valve) - which is really really bad, and you will end up with a kite with "almost" sufficient air only
So one-way valves has been tried by some, but I think they have dissapeared again because of above
I dont know, as it might be very kite specifific - just a general thought