Two antiviruses on the same PC (more precisely: on the same OS installation) can bring real troubles: all antiviruses use the same techniques for malware detection (e.g. file tracking, interception of system calls, stacktrace analyzing, etc). So activity of the one of the antiviruses can be considered as intrusion actions by another antivirus. You should uninstall one of them anyway.
I tried to boot into safe mode but it's not possible. I don't
understand the possible reason behind it.
I haven't reproduced your situation (I just haven't Windows box). But most of the modern antiviruses start functioning at the very beginning of OS boot process (this is intended for fighting some kind of sophisticated viruses). Perhaps, this is why you can not boot into safe mode.
Could it be due to multiboot system?
No. BTW what bootloader is on your PC? Is it grub or something else?
I suggest you to boot from LiveCD (or enter to bootloader menu, for example, in case of grub just hold Shift at boot time). Then you can uninstall on of the antiviruses. For example, you can try to remove the antivirus' directory from Program Files folder. Yes, it's dirty uninstallation. But it will stop antivirus. Please look at the documentation of antivirus vendor for details (probably, there is official uninstallation tool...).
IMPORTANT! Don't delete any files permanently. E.g. load from LiveCD, plug in an USB flash drive. Then, for example, do not just delete antivirus directory from Program files but move corresponding directory to flash drive. If something will go even worse (for example, Windows will not boot at all) you can easily restore the files. So you will able to try another approach.