Multiple virtual bootable 'disks' in one USB flash disk

I have several 'emergency' tools, focused on hard disk and system failures.
All these, should be able to boot a mac, in case of a problem.
Several bootable utilities, each one claiming it's own 'media' to boot a mac.

The problem I face is that all new mac do not have dvd drives, and also I want to detach from them.
So, what option do I have? Usb flash disks or similarly, several sd cards.
Unfortunately, I cannot afford having a dozen of them.
I cannot afford dedicate a separate usb flash disk for each one of these boot utilities.

I would like to be able to use a flash disk in any way I want(daily tasks, transferring data etc), but in case of a mac problem, I want to be able to use it as a bootable device too.

Is this possible? Can I do it and how?
I tried copying a specific bootable dmg in a folder of the root of my flash disk, I even tried and copy-paste in other folder all the files(without the dmg image) of the image, but when I booted mac pressing 'alt' button, I could not see any option of alternative booting.

So, how can I do it?
I hope there is a solution, other from purchasing many usb sticks or reverting back to cheap dvd disks.

Can I make partitions to a usb stick(not hard drive), with Disk Utility?

And then mount them and restore every bootable dmg package I want, to a separate partition of my usb stick?

I used disk utility and made several partitions to my usb stick.
2 partitons 'mac journaled', and 3 with 'ntfs'(tuxera).
When I plug it to mac, all partitions are visible.
When I plug it to windows, only one 'ntfs' partition is visible, the first one, and no mac partitions are visible.
I thought I could see all three partitions with ntfs format, inside windows.
Why this does not happen?

(I hope to solve it here, because if I ask to a pc forum they will blame me for creating it using mac's disk utility. But after all, I want a solution either using pc or mac. Pc users do not usually know mac stuff, so I hope some mac users with windows experience to help me here. It is a cross platform problem).

If I choose to make partitions in windows, I can't make mac formated partitions!
So, the only way is to make it on mac!
But, I will use usb stick to pc too.
So I need windows and mac format in several partitions.
Why there is such problem?