Safe way to partition

My story:

I have snow leopard running on my new 27in iMac. I use old versions of software that don't work with Lion. I have a 1TB hd in my mac and a 2tb external hd I have all my Time Machine backups on. I want to update my Mac HD to Lion. Then partition the 2TB external drive and use 1TB for Snow Leopard and 1TB for Time machine on my Mac. This way I will always have a Time Machine backup AND still be able to use all my old Power PC apps.

What is the best possible way to do this AND have all the same data on each OS?

Thanks