With AC, you will get a loud bang if two wires from different phases touch. Nothing will happen though if the wires are connected to the same phase.
AC Voltage measurement is done between phase & neutral OR between phases. So you get 220V when you measure voltage between phase & neutral and 440V between two different phases. There is no +ve or -ve in AC supply.
With DC, you will not get any reading if you do not measure between +ve & -ve. However if the voltage sources are different then it depends on how these sources are connected. If DC sources are connected in series, then you get added up voltage (two 1.5V cells make 3V) and if you connect them in parallel, the voltage remains the same but the current is added up.
Voltage is also called "Potential Difference" i.e. the difference in potential between the terminals. It is always measured between two different terminals.