Unetbootin dual boot failure

Dear users, I checked the old topics but I found no matching for my problem.

I tried and set a live USB stick with two Linux distros on it (Puppy 6.3.2 and Linux Mint 17.3). Pendrive is a Cruzer Edge 8GB.
I partitioned the drive into 3 (partition 1 for Puppy, partition 2 for Mint, partition 3 for data) and I tried multiple filesystems configs (1° - partition1: Primary-ext4, partition 2: Primary-ext4, partition3: Primary-extended. 2° - partition1: Primary-ext4, partition 2: Primary-ext4, partition3: Primary-ext4. 3° - partition1: Primary-ext3, partition 2: Primary-ext3, partition3: Primary-ext3. 4° - partition1: Primary-FAT32, partition 2: Primary-FAT32, partition3: Primary-FAT32). So the reformatting to FAT32 has already been done. The failure result is filesystem independent.

I run Unetbootin linux version (attempts with the 32 and 64 bit version ) both from a Puppy 6.3.0 or Knoppix 7 live CD and Unebootin worked fine leading to a successful installation of the ISO 1 (Puppy) on partition 1 and of the ISO 2 on partition 2.

The problem arises when I boot from the USB stick. On the first Unentbootin screenshot there is no dual boot manager option: either I have a boot for slacko Puppy 6.3.2 or a boot for Mint 17.3. I checked the partitions on the Cruzer Edge and I noticed that if partition 1 is labelled as bootable, Unetbootin bootloading installation just detects partition 1 and shows just Puppy boot options, if I force partition 2 to be labelled as bootable, Unetbootin bootloading installation just detects partition 2 and shows just Mint boot options.

How can I make detectable the second non-bootable-labelled partition to the boot manager and how can I actually make a boot from a non-bootable partition?

I am new at Linux. so forgive me if this question has been already answered somewhere else.

Thanks a lot for any help.