You want to send the entire message each time a single element is copied? You want to copy from myArray1 and then immediately overwrite it with data from myArray2? It's not hard to see that this code does that.
void loop(){
for (i = 0; i < 20; i = i + 1) {
data[i]=myArray1[i];
data[i]=myArray2[i];
radio.write(data,sizeof(data));