Say, you want to pin a sword to character's hand.
Create an image point on the hand, name it "SwordPoint".
To pin the sword, first position the sword to "SwordPoint" (using "Set position to another object" action), then pin.
When you mirror the character, unpin the sword, mirror the sword, set its position to "SwordPoint", then pin again.
Another way to do this is to move the origin image point on the sword in such way that when you position it to character's origin image point, it appears in its hand. Then you'll be able to simply mirror both the character and the sword.
See this picture:
red dot is the character's Origin image point.
red arrow shows the sword's Origin image point.