The problem is that /dev/sda contains /dev/sda2. This would be a severe
obstacle if both should be mounted as read-write filesystems.
But in your case the ISO 9660 in /dev/sda (also in /dev/sda1) is read-only.
So there will be no fighting of filesystems.
The normally correct way to have both filesystems mounted would be to
umount /dev/sda and to mount /dev/sda1 instead. It does not overlap with
/dev/sda2. So mount(8) will perceive no problem
But if the running system depends on files in the ISO 9660 filesystem,
it might not be possible to umount it.
In this case there remains the backdoor of using a loop device like
/dev/loop0. Such a device is based on a data file or block device file.
It acts as block device. See man losetup(8) for details.
mount(8) offers the convenience option "loop". So this should work although
/dev/sda is already mounted:
mount -o loop /dev/sda2 /your/mount/directory
Luckily mount(8) is too stupid to recognize the overlapping although it
then lists the partition device as mounted rather than the loop device.
/dev/sdc on /mnt/iso type iso9660 (ro,relatime)
/dev/sdc2 on /mnt/fat type vfat (rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=utf8,shortname=mixed,errors=remount-ro)
So this trick might cease to work in future ... :(
(But maybe it also gets smart enough to recognize that the overlapping