After the connection is made, they will be at same voltage and freq.
Other factors will determine direction & magnitude of energy flow.

The BIG problem is in parallelling them before making the connection.
If not done, there can be one serious electrical/mechanical jolt on
making the connection. You need to get phase-angle to zero, and one
traditional way of sensing this is with light bulb connected across
phase leads before connecting the leads.

HTH,
J