Send: 0 0 1 1 (= Write to and update DAC Channel A) + 0 0 0 1 (= DAC A) + DAC data ( = my 2 bytes).......
How do I build that byte array
No that is what you need to send in the first byte it is not a byte array just a byte.
The top four bits of the first byte is the command you want to do which in your case is 0 0 1 1 so if that is in a variable you would say
command = 3;
and
what you want to do it to is dacA
dacA = 1;
Now to join them together shift the command by four places to the left and OR it with the dac like this
send = (command <<4) | dacA ;
Or just write it out as 0011 0001
send = 0x31;