Stockdale, who pled not guilty by reason of insanity to the murders of his mother and brother and is set to stand trial next month, failed twice in his attempts to escape the institution ... according to his treatment staff.

In court docs, obtained by TMZ, during a hearing on Stockdale's competency to stand trial, the staff noted Stockdale's first escape attempt occurred in November 2019, when he allegedly tried hiding between book stacks in the mental hospital's library, leaving staff concerned he was trying to find a way to exit the building.

Stockdale allegedly tried to escape again a month later ... the staff says this time he tried blending in with a group of people who were on their way out the door.

Ty Graham, one of Stockdale's attorneys, tells TMZ ... Stockdale was not charged with attempted escape.

As you know, Stockdale's case is gruesome ... back in 2017, he was arrested for shooting and killing his mother, Kathryn, and his younger brother, James. Stockdale then shot himself in the head, but he survived and underwent months of reconstructive surgery.

The Stockdale family appeared on "Wife Swap" way back in 2008 ... and they're remembered for living by strict rules and being extremely religious.