Hi. I already used these boot images but I was still having a problem. I had flashed the stock image to both partitions and then the Magisk patched image also but either I was not rooted or Magisk App not working. Finally got things to work by installing TWRP to partition a I believe. I changed active partition to a and then flashed the twrp recovery. Again changed active to b and booted and had root. I had to install Magisk Canary 25210 version only as other versions did not install. After checking on net it seems this version is faulty. So I installed Magisk Delta, then installed Magisk using Direct Install option, rebooted, checked regular Magisk Manager and showed the same Magisk version. Removed all modules from old Magisk Manager and unstalled the app. Setup Magisk Delta modules, hide, SuList as well as apps like LSPosed, HMA etc and now everything is working again.
I still am not very sure where the recovery is installed, on partition a or b? And if a difference recovery is installed on each partition what happens? I am pretty sure I had not installed TWRP on partition b. I am also assuming that once I set a partition as active and flash a recovery in fastboot that recovery is flashed to that partition, is this correct? Things are working but I don't know if the phone will go to recovery mode from system, I have not tried that. So I am not very clear yet but still got things to work