You can only be in one space at a time. There are too many variables to consider when given the possibility of multiple spaces simultaneously, like monitor size, orientation of screens and spaces, and what number to actually display in the menubar for the current space (if you use that feature).
What you can do is assign two spaces worth of applications to one space and just arrange them in the two monitors. On 10.6 and before, I did this by having a space layout of four rows by one column, using two monitors side by side. Moving up and down by space changed the content in both monitors simultaneously, which was very usable.
Lion is a little limited in terms of flexibility between multiple monitors and spaces, and I anticipate that this will get more streamlined with future updates, but it's actually a little more like having a different space for each monitor at this point. When you activate Mission Control, you should see different spaces at the top of each screen.
Unfortunately, I don't think you'll find the flexibility you're looking for when changing numbers of monitors. If you set yourself up with two and assign different applications to different screens within one space, they will likely collapse into one screen when you detach the monitor.