Handerson Hasselbach equation:

pKa of phosphoric acid for pH 6 is 7.21

pH = pKa + log ([A-]/[HA])

Now, for A- you put Na2HPO4 concentration, and for HA you put KH2PO4 concentration:

6 = 7.21 + log(A/HA)

log(A/HA)=-1.2

A/HA = 0.063

then, HA+A=0.05 also A/HA=0.063

HA=0.047M

A=0.003M

in one liter of solution.

for molecular weights: KH2PO4==136; Na2HPO4= 23*2+96=142Thus, 0.047*136=6.392g of KH2PO4 and 0.003*142= 0.426g of Na2HPO4 should be added.

Thank you very much Dr. Hatem.  That was very helpful.