You could run Deskew in detection only mode and then rotate both images in some other tool.

If by cmd you refer to windows then

@echo off
deskew -g d file1.png|find "[deg]" > out1.txt
deskew -g d file2.png|find "[deg]" >> out1.txt
type NUL > out2.txt
FOR /F "tokens=1,2* delims=:" %%A in (out1.txt) do echo %%B >> out2.txt

gives me in the out2.txt file

 -0.310
 -0.390

Note I have not tidied up the leading space nor added any other averaging to get perhaps a common value of -0.350
Since cmd has exceptionally poor integer math you need to move decimal points three digits right, do addition and divide by 1000
so in this case roughly we strip the decimal points and any leading zeros and using variables would attempt

setlocal ENABLEDELAYEDEXPANSION
set /A "V1=-310"
set /A "V2=-390"
set /A "Avg=(V1 V2)/2"
echo !Avg!

result is -350 as expected so needs to be returned as -0.350