Format the second partition as FAT32 and you're set. Kinda...
You'll probably want to use a large boot partition (first partition). See below.
Windows is weird about partitions on removable media, you may need to use the raspberry's boot partiton as your cross platform partition as the raspberry only boots from the first partition, and Windows will only allow access to the first partition... (unless you hack the disk driver inf file or something like that)
Luckily the boot partition must be FAT32. So windows won't have any problem reading and writing to it. Just be careful not to clobber your boot files...
I use Arch Linux ARM on my Raspberries, I like the flexibility and ease of install. If you want a Windows compatible partition that just works, try this:
Boot a Linux live CD or use a Linux system.
Partition the SD Card from a machine like this:
Parition Table: MS-DOS MBR
Partition 1: 1GB FAT32
Partition 2: 7GB(remaining) BTRFS
Download Arch Linux ARM for the Pi
Raspberry Pi: http://archlinuxarm.org/os/ArchLinuxARM-rpi-latest.tar.gz
Raspberry Pi 2: http://archlinuxarm.org/os/ArchLinuxARM-rpi-2-latest.tar.gz
Mount the second partition, create the boot directory, then mount the first partition as the created boot directory.
Then untar the downloaded file to the root of the SD Card's second partition.
Modify the cmdline.txt file like this:
root=/dev/mmcblk0p2 rw rootwait rootfstype=btrfs rootflags=rw,relatime,ssd_spread,space_cache console=ttyAMA0,115200 console=tty1 selinux=0 plymouth.enable=0 msc95xx.turbo_mode=N dwc_otg.lpm_enable=0 kgdboc=ttyAMA0,115200 elevator=noop
Then boot the Pi from the SD Card ans you're set to configure the system from there.
Windows should also auto mount the first partition (this boot partition) and you should be able to read and write files there.