Note: I've answered the question as asked, but what you probably want is my last paragraph.
Manual partitioning and dual boot configurations
You'll need to install from the alternate CD for dmcrypt. Select manual partitioning in the installer. If you want two separate encrypted volumes, make them both primary or extended or EFI partitions, and leave enough room for the other volume when you install the first OS. If you want two Linux distributions that share an encrypted volume (but you probably don't want that as I explain below), create one large encrypted volume and create the filesystems for each OS when you install that OS.
Make the encrypted volume an LVM physical volume and create a volume group that spans just that physical volume. If two operating systems share the encrypted volume, they'll also share the volume group: volume groups are a way to manage multiple volumes, and in that configuration there's just the one. Create at least one logical volume for each root filesystem. If the two OSes share the encrypted volume and you don't want to use hibernation, they can share the swap area.
Leave a large enough boot area, which needs to be unencrypted. I recommend at least 1GB (lets you store a rescue CD in an emergency) unless you're heavily constrained on disk space, but 200MB is enough for just one kernel and one alternate kernel.
Having two separate boot partitions is the simplest way to install. Let one distribution control the boot sector, and install the secondary distribution's bootloader on the first sector of its boot partition. Chainload the secondary distribution's bootloader from the primary bootloader.
If you want a single boot partition, install Grub only on one operating system, and share
/boot between the two operating systems.
Installing two Linux distributions
Dual boot is painful to use, and not needed in most situations. If you want to run two operating systems, use some form of virtualization. The only use case I can think of for dual boot is when you need to test multiple operating systems on the same expensive hardware (even then, if you can afford it, it's more convenient to have separate machines per OS).
You will need dual boot if you want two separate Linux installations with different passwords. Even then, you may well be able to implement your desired security policy by issuing separate accounts or by creating separate virtualized containers, each with its own root users.
To run Linux on Linux, you may not even need virtualization: you can run programs from another installation with chroot. With Ubuntu or another Debian-based distribution as the master OS, schroot makes this very easy. If you use schroot with a different secondary distribution, you'll probably need to install it separately, so on a separate volume. If the secondary distribution is Debian-based and you never need to boot it, you can install it in a subdirectory with debootstrap. See my schroot guide.
Ubuntu/Xubuntu dual installation
If you want to try out both Ubuntu and Xubuntu, you don't need all this rigmarole. Xubuntu and Ubuntu are the same distribution with different default packages. Install Ubuntu from any variant and make sure both the ubuntu-desktop and xubuntu-desktop packages are installed. Select your preferred environment when you log in, and you will be using either Ubuntu/Unity aka Ubuntu or Ubuntu/XFCE aka Xubuntu.