Term Clarification

When left and right clicking at the same time, you have to left click slightly earlier than the right click, since when you right click first and hold, it will disable the ability to left click. So left click first, and then right click, then hold. I'll call this action "hitblocking."

The Bug

When you hold 2 tridents at the same time and hitblock, the offhand will turn slightly outwards. When you shift while in this state, the back of the trident will go through the player's body. 

When you hold a riptide trident on the mainhand and regular/loyalty one on the ofhand(MC-136822) and hitlock, the player will hold the trident on the offhand backwards. 

This bug also happens when flying with the elytra.

You can observe this glitch easier when you give yourself mining fatigue. 

https://gfycat.com/AnxiousPlayfulEarwig