can open multiple ArcGIS Map Document(.mxd) ?

Hi ,
I would like to know that I can open multiple ArcGIS Map Document(.mxd) simultaneously or not ?
The scenario is like the following :
I opened the .mxd file with "Open Map Document" command on the toolbar and create layer . After that add an image on the layer.
For second map document , I do the flow as above.
After that I would like to switch to previous MapDocument(.mxd) withoot using "Open Map Document" but just using "Switch .."
although everything is fine when I alter back to first map document , the image on the second layer of second document that I added is missing when I switch back to second map document.

To be concluded, I would like to know that multiple MapDocument(.mxd) can open simultaneously and work or not ?
Thanks in advance

Hsu