Problem with input source changing by the cmd and Dell Display Manager (ddm.exe)

I have two Dell displays.


I want to switch input source for 24'' from the my operatin system level (WIndows 8.1).
I'm using Dell Display Manager from command line like this:

ddm.exe /2:SetActiveInput DP (as my second monitor):


Now the problem is, that COMMAND LINE once working for "2" as a number of the 24'' monitor and once not.
24'' input source change once is working for "2", other once for "1".

How can I change it properly?

Is there any possibility to choose the ID of a monitor in deterministic way?

Example of usage:

COMPUTER 1 WITH HDMI1 c:> ddm.exe /2:SetActiveInput DP (working)

COMPUTER 2 WITH DP c:> ddm.exe /1:SetActiveInput HDMI (working)

COMPUTER 1 WITH HDMI1 c:> ddm.exe /2:SetActiveInput DP (not working)

COMPUTER 1 WITH HDMI1 c:> ddm.exe /reset
*must wait a while*
COMPUTER 1 WITH HDMI1 c:> ddm.exe /2:SetActiveInput DP (working)

Why numbers are changing after input source switching?

Windows keeps same numbers for all monitors all the time.