Is it possible to do Terminal Services RemoteApp between two XP Pro machines?
This is impossible. Terminal Services RemoteApp requires... Terminal Services which is a component of Windows Server and only Windows Server.
If you want to use an XP machine to deliver a remote application to another XP machine, you will need to use a third party product such as TSPlus. You will be limited by the number of simultaneous connections that are allowed to an XP machine (that being 10). You will also be restricted by the inherent unreliability of a desktop operating system.