Can only open one copy of PoE with dx11

Using dx9 or dx9ex I can open 2 instances of PoE on one computer; however, if I try dx11 I get the error message: "You may only run up to two copies of PoE at one time." When I close that dialog I get the message: "The dx11 client crashed on startup. would you like to switch to dx9 client instead."

This error occurs on both my windows 8 pc and windows 10 laptop.

Confirmed on Win7 x64 trying to run both Standalone and Steam versions of 2.5.2c.

I obtain the same error dialogs as described in the OP.

Occurs trying to run two copies of directx 11 client;
or when trying to run directx 11 client and a second directx 9ex client*;
but I can run two directx 9ex clients without crashing.

Bug is still there on version 2.6
This is because the 32bit executable acts as launcher for the 64bit one, and the latter considers the "launcher" as another copy of the game.
Workaround: for the second instance launch directly the PathOfExile_x64.exe executable from PoE folder.