Can the fluid from one hydraulic circuit go to another one (voluntarily or not) in flight?
This depends on how the systems are plumbed, and while this is likely an exception, for some airplanes like the EMB-145, the answer is yes.  

The EMB-145 has 2 hydraulic systems.  System #1 serviced the outboard brakes and system #2 serviced the inboard brakes and the parking brake.  The parking brake actuated all of the brakes and this let you transfer fluid between the systems through the outboard brakes.  This isn't a normal procedure, but one maintenance would walk you through if you had low fluid in one system and more than you needed in the other to balance the systems if you were out somewhere without the ability to service the hydraulic fluid.

The procedure was either 


Depress and hold toe brakes
Set parking brake
Release toe brakes
Release parking brake


or the opposite, depending on which way you wanted to transfer fluid.

This works in flight or on the ground (don't do this in flight lest you get distracted and end up landing with the parking brake set)