You still can parent hand to hand of 2 figures but at least one of them must have IK (the one which is going to be parented must be IK Goal - it will appear in: Figure>Use INverse Kinematic menu if it is).

Now switch it on (IK for that hand) and you'll see that set parent button for that hand becomes avaible. Press it and choose hand of other figure to parent to. Done.

Every time you disabe IK for the parented hand the connection between child and parent (those 2 figures I mean) will break. To bring it back just swich IK on again.