It is certainly possible to implement, as far as I know, and is definitely one of the changes that is planned to be incorporated in iLiad OS one of these days. However, it's going to be a while (months) before it comes up in the development timeline. If you (or someone) wants to take a crack at it, that would be awesome. Currently that is my personal #1 most-wished-for feature.
If anyone is interested in working on this I would suggest first looking at the contentLister source code. The contentLister is designed to close one program before opening another. Presumably this behavior could be changed to allow several programs to be open at once.
We had a few discussions of how the UI for multi-mode could be implemented to make it as intuitive as possible...
here's one, but I think there was another one too...
Another issue that might need to be addressed is memory. The iLiad doesn't have a ton of RAM, and I know that iPDF will take as much memory as it can to store pre-rendered pages. There might need to be a way to limit how much memory each program is allowed to use.
Anyway, as far as I know this feature does not yet exist, but it would be really nice to see.