I don't know but I would not do it this way.
1 - use Carbon Copy Cloner (or some other means) to create a bootable clone of the internal disk onto an external disk
2 - test the clone for use and bootability etc
3 - erase the internal disk
4 - split the internal disk into at least 2 partitions
5 - install complete OSs on each partition
6 - experiment with sharing data only between partitions but expect difficulties with iPhoto and iTunes
Yes, a number of people, including myself, did that when Lion came out. On your external hard drive, create a dedicated partition for your Lion system. You want to keep your Snow Leopard Time Machine backup separate from Lion, hence separate partitions.
After using Lion for awhile, I created two partitions on my internal hard drive, one Lion and one Snow Leopard. I use Lion for everyday use, and only use Snow Leopard to now and then run a PowerPC program or two. You probably know that Lion dropped emulator support for PowerPC programs, so they no longer run under Lion.
And you probably also know that Lion requires a minimum of 2GB of memory, however, 4GB is recommended.
I created the bootable clone and verified it's operation. Then it sounds like you're talking about a "dual OS boot" with two partitions on the internal drive. If that is correct, where does the data get stored for both partitions to access it?
You should be able to access data across partitions. You can probably also access applications across partitions but I would not advise it.
It can sometime be useful to have data on both partitions as a protection against some mishaps although this will not help if the disk fails.
You can probably also access applications across partitions but I would not advise it.
This was badly stated. I should have said:
It is normally OK to access applications across OSs but this can cause problems if the OSs are different versions or, in a few cases, if the applications are badly written and make assumptions about their location. Many applications assume any preferences or data they use reside in locations relative to the user's Home folder and this aspect may require some thought.
Occasionally the OS does its own thing and uses an application on the wrong partition. This can be annoying but I have never known it to result serious data loss.
It is technically possible to have different partitions for OS, applications and user data (Home folders) etc but the average user would not welcome the complication which this can cause.
My main 120GB disk has 8 volumes. Some with different fully functioning OSs and some with various modified OSs and installers. In addition to this I have several 750 GB disks for backup.
One nice thing about an extra bootable partition is that it can be used to repair another partition as long as Disk Utility is compatible with it. This can be a useful alternative to using the installer disk or AppleJack. Some recent OSs have hidden partitions with such facilities.