Do you use two mics for convenience of standing vs sitting, or for different e.q. for backup vs lead vocals? 
I used two mics so I could you the VH on each keyboard. I tried using a stand alone VH with a unit called the Midi Merger but discovered that it doesn’t work the the Yamaha PSR 950. 
Eventually I’d like to just  use one KB, but I haven’t  found a good solution. Most gigs I just use one keyboard but when there’s both lots of Mexican , tropical  Latin and “American” , I use both keyboards.