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Is there a method to create a multiboot USB with both Windows and Linux ISO files using a Linux machine? 
(Specifically, I'm on Arch Linux.)
I tried multibootusb, multisystem, yumi, syslinux, etc., but couldn't find a clear guide on having both types of ISOs; almost all the guides had one type of ISO and Windows tools. 
How can I approach this?

You could try the following:

Use WinSetupFromUSB

• Download WinSetupFromUSB.


• Extract this file.


• Connect the flash drive to your computer.


• Open WinSetupFromUSB. Open the 32-bit or 64-bit version according to your OS. It will work without any installation.


• Make sure your flash drive is listed and selected in the drop-down menu. Click Refresh, if it is not on the list.


• Tick Auto format it with FBinst.
  
  

  
  

  Note: You have to tick this option only for the first ISO installation. Choose FAT32 if your computer is set to boot using UEFI mode or if UEFI sounds strange to your ears. Else, go with the NTFS option.

  
  

  
  


• Click Advanced options.


• Tick the Custom menu names for Vista/7/8/Server Source checkbox. Click the Cross (X) button to exit Advanced Options.


• To add the ISO file for your multiboot USB, tick the checkbox corresponding to OS under Add to USB disk subheading.
  For example, I am using the Windows 8.1 ISO.
  
  

  
  

  Note: If the size of your ISO is larger than 4 Gb, it will show a message to split the file into parts. This is because you’ve chosen the FAT32 option. Click OK.
  Note: WinSetupFromUSB doesn’t support dual ISOs i.e. it can’t have 32-bit and 64bit version in a single ISO. It will show an error message.

  
  

  
  


• Click GO. A data deletion warning message will be displayed. This is because you’ve chosen to format the flash drive. Click Yes.
  Note: Do remember to check the flash drive name before clicking Yes. Else, you’ll end up formatting some other connected storage media.


• Another warning message will be displayed telling you that all the partitions will be erased. Click Yes.


• Next, it will ask for the folder name. Type the desired one in 30 seconds or it’ll auto select. Click Ok.


• It will ask for the boot menu name. This will appear when you run the multiboot USB on some PC and choose an operating system. Type your desired name like Windows 8.1 64-bit. Click Ok.


• The process will take few minutes to finish.
  Click EXIT to finish.
  Then Add the next ISO

Rest of Istructions Here:

https://fossbytes.com/how-to-put-multiple-iso-files-in-one-bootable-usb-disk-create-multiboot-usb-disk/.