This seems to be the official way. http://wiki.centos.org/HowTos/InstallFromUSBkey
Some live cd to usb type creator might work with live centos too.
There may be a few ways. I haven't tested this but grub4dos may be able to directly boot to one iso image. Then when asked to find the second it might show up on the usb or you may have to create a second partition on the usb to find it.
Sometimes network installers can be booted on a usb and then look to either the files extracted to partitions on a usb or the iso image.