I have a very similar setup and the same problem. Interestingly enough, the problem goes away if I configure Monitor1 as "RightOf" instead as "LeftOf". Then the sddm greeter is on the primary screen, and the other one stays black.
$ cat /etc/X11/xorg.conf
--- snip ---
Section "Monitor"
Identifier "Monitor0"
VendorName "HP"
ModelName "w19"
HorizSync 30.0 - 83.0
VertRefresh 55.0 - 75.0
Option "DPMS"
Option "Primary"
EndSection
Section "Monitor"
Identifier "Monitor1"
VendorName "Dell"
ModelName "P190S"
HorizSync 30.0 - 81.0
VertRefresh 56.0 - 76.0
Option "DPMS"
Option "RightOf" "Monitor0"
# FIXME this should be:
# Option "LeftOf" "Monitor0"
Option "Rotate" "CW"
EndSection
Section "Device"
Identifier "Device0"
Driver "radeon"
VendorName "Radeon"
Option "AccelMethod" "XAA"
Option "AccelDFS" "1"
Option "EnablePageFlip" "1"
Option "ColorTiling" "1"
Option "Monitor-HDMI-0" "Monitor0"
Option "Monitor-VGA-0" "Monitor1"
EndSection
Section "Screen"
Identifier "Screen0"
Device "Device0"
Monitor "Monitor0"
DefaultDepth 24
SubSection "Display"
Depth 24
Virtual 2464 1440
EndSubSection
EndSection
/ Knut Esztermann
--
Linux - Less bugs for less bucks!
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