Boot the MultiBootUSB drive.
Open Nautilus as root, go to computer/cdrom and make a folder named Shared Data.
Save any stuff you want to share with a Windows computer there, You will need to be root to access the folder.
When plugged into a computer running Windows, the Shared Data folder will be accessible when the drive is opened in Explorer.
Windows can only see the first partition on a flash drive and MultiBootUSB needs to be installed on the first partition, but they can share it.
Mkusb will create a Ubuntu flash drive that boots using grub2.
It uses a FAT32 boot partition, a ISO9660 read only OS partition, an ext casper-rw persistence partition, (not limited to 4GB), and a NTFS partition that can be used by Linux or Windows.
https://help.ubuntu.com/community/mkusb
You can split the casper-rw persistence partition into casper-rw plus home-rw if you want more persistence options.
It is a little work to turn a mkusb drive a multibooter but it can be done by editing grub.cfg to boot iso's located in the usbdata partition.
Up to 14.04 it was possible to add a persistent partition to syslinux type Live drives such as Startup Disk Creator and Unetbootin make, now days this just results in BusyBox.