It's a bit puzzling that HDMI-A-2 shows connected status but edid is not being read - please run "pastekodi" and post the URL.
It might be worth forcing connector status to enabled (eg with video=HDMI-A-2:D) and double-checking your cmdline is correct.
eg this should work (using lg-55c8.dat edid on first HDMI, forcing mode to HD and your pioneer edid on second HDMI forcing to always on without specific mode - you could also use 1920x1080@60D there):
drm.edid_firmware=HDMI-A-1:edid/lg-55c8.dat,HDMI-A-2:edid/pioneer-vsx520.bin video=HDMI-A-1:1920x1080@60D video=HDMI-A-2:D
Testing that here (with nothing connected to second HDMI) worked fine.
EDID loaded lines in dmesg:
[ 3.434129] [drm] Got external EDID base block and 1 extension from "edid/lg-55c8.dat" for connector "HDMI-A-1"
[ 3.444927] [drm] Got external EDID base block and 1 extension from "edid/pioneer-vsx520.bin" for connector "HDMI-A-2"
and kodi also detected the audio device with the pioneer AVR
2021-03-21 13:22:04.378 T:948 INFO <general>: Device 6
2021-03-21 13:22:04.379 T:948 INFO <general>: m_deviceName : hdmi:CARD=vc4hdmi1,DEV=0
2021-03-21 13:22:04.379 T:948 INFO <general>: m_displayName : vc4-hdmi-1
2021-03-21 13:22:04.379 T:948 INFO <general>: m_displayNameExtra: PIO VSX-520 on HDMI
2021-03-21 13:22:04.379 T:948 INFO <general>: m_deviceType : AE_DEVTYPE_HDMI
2021-03-21 13:22:04.379 T:948 INFO <general>: m_channels : FL, FR, LFE, FC, BL, BR, BLOC, BROC
2021-03-21 13:22:04.379 T:948 INFO <general>: m_sampleRates : 32000,44100,48000,88200,96000,176400,192000
2021-03-21 13:22:04.379 T:948 INFO <general>: m_dataFormats : AE_FMT_RAW,AE_FMT_S24NE3,AE_FMT_S24NE4,AE_FMT_S32NE,AE_FMT_S16NE,AE_FMT_S16LE,AE_FMT_S16BE,AE_FMT_U8,AE_FMT_RAW
2021-03-21 13:22:04.379 T:948 INFO <general>: m_streamTypes : STREAM_TYPE_AC3,STREAM_TYPE_DTSHD,STREAM_TYPE_DTSHD_MA,STREAM_TYPE_DTSHD_CORE,STREAM_TYPE_DTS_1024,STREAM_TYPE_DTS_2048,STREAM_TYPE_DTS_512,STREAM_TYPE_EAC3,STREAM_TYPE_TRUEHD
so long,
Hias