Is it possible to keep many currencies (Bitcoin, Monero...) on one USB drive? If not, how can I setup USB Cold Storage for Bitcoin and Monero?
Neither currencies nor coins are stored anywhere but at addresses out on the blockchain.
You may be talking about the secrets that give authority to send / spend those assets?
Seed Recovery phrases, passwords, PIN codes, private keys, master extended keys...
These are just data, text and/or numbers.
So of course millions of such data can be stored on a USB.
"Cold storage" just means, not needed for spending, kept away from any devices capable of connecting to a network, no wifi no bluetooth. Could be on a USB, or written on a paper or steel sheet kept in a safe deposit box.
Then you have the "wallet" data files or proprietary backup files as used by some client software, or a hardware wallet device.
In order to get good advice, please rephrase your question so we have more detail about your situation and just what you are trying to accomplish.
The easiest way is to write down your private keys in file on a USB drive.
Here some instructions for Monero (you can do the same thing for Bitcoin as well):
Yes. A few different options.
Easiest: Save your seed and private keys into a file onto your USB drive. Then pull it out of your computer. It is now in cold storage.
Not as easy but works: Buy a laptop (or rasp pi) and save everything onto this. Then disconnect from the internet. Boom, cold storage.
My favorite: Make a portable, bootable USB drive running Linux. This is a portable operating system that you can boot into when you start your computer, that would be separate from your primary operating system. You can then install your wallets onto this portable OS, and when you are done, power down, and pull out the USB drive. Then the wallet and all the keys are on this USB and in cold storage.