You would have to mount the opensuse.iso to a mount point, then copy the .iso to a temporary (empty) folder for editing. In /boot/x86_64 (or the appropriate folder if using x86), look for file mkbootdisk. I suspect the answer is in this block of code:
my $part = $opt_disk;
$opt_disk =~ s/(\d+)$//;
my $pn = $1;
die "not a partition: $opt_disk\n" unless $pn ne "";
$opt_disk =~ s/(?<=\d)p$//;
print "disk $opt_disk, partition $part\n";
die "sorry, must be a primary partition (number 1 - 4)\n" if $pn < 1 || $pn > 4;
Mkbootdisk is a perl script, which I read about as well as I read Chinese. Notice references to $part, which I think is the partition number to be searched for the isolinux loader. Also note that $part refers to $opt_disk. If you can figure out how to change that to search the second partition, you can then:
1) umount the .iso
2) using the unpacked .iso in the temporary folder, make a new .iso.
3) put that new .iso on the USB stick.
I hope I haven't led you astray.