I don't think this is possible:
1. A project can only have one memory configuration. 2 memory configurations = 2 projects
2. Even if you could change memory configs (which you can't), you cannot build 2 targets in 1 build. 1 build = 1 target
Ideas on how you might make this a bit easier.
1. Set up two projects. Use linked resources (described in online Help) so they both use the exact same source.
Or
2, (if the ONLY difference is the link step memory map), have 3 projects - one builds a library from your source, the other two have the separate memory configurations defined