Connecting AC and DC Voltage Sources in Parallel
Forget it. It's a short circuit. The current between the sources will become as high as the internal resistances of the sources and the wire resistance allow. In pure theory with ideal parts the current would be the voltage difference divided by zero Ohms, which we call infinite.
The problem vanishes if you have 2 resistors. Insert one resistor in series with each voltage source. Then you can connect them together for ex. to make a mixer for 2 signals.
Not asked: 1) Connecting AC and DC voltage sources in series is a common way to present a signal which has DC-component - a nonzero average. 2) Connecting in parallel current sources has no theoretical problems, but connecting 2 different current sources in series contains the same math impossibility than connecting 2 different voltage sources in parallel.