One PSU powering one high power device & one low power device
Your question is similar to this example:

The water tower in my town can supply 76 households simultaneously. If 40 households has their shower running, will I, number 41, also get water when I take my shower? Or is the water tower unable to give me water?



Extra information: 


\$19.5V×3.9A=76W\$
\$5V×2A=10W\$
\$4\$ buck converters => \$40 W\$ 




Yes, they will take their showers, you are not going to take their water, nor will they take your water. 

You could even attach 3 buck converters more and have 6 W to spare for the Arduino nano. The Arduino nano will not consume 6 W, it will take what it needs. The remaining W that is not taken stays at the water tower, it stays in the electrical grid.