If it can be done, (see below), it would be a peer to peer network. You would install denplan on both PC's and then change the settings on one of the PC's to make it use the data files/data source on the hard drive of the other PC, which would need to be shared on the network.
It all depends upon how denplan works. There are a number of questions you need answered, either from this forum or the writers of the denplan software:
Has denplan been written to be a single-user or multi-user application? If it is single user then you will not be able to have simultaneous access of the data files (withough risking corruption). If it is multi-user you still may not be in the clear as you may need to buy a second licence, it depends if the writers have done anything stop you setting up a second user.
Are the appointments etc. stored in a database and if so what kind of database and how does the software access it?
If you need to work this out yourself. To begin with, I would look at a working single PC installation. Is there a tools/propeties/options menu tab? Can you determine the directory where the data files/database is stored on this PC? Is there a printed manual or .PDF that maight help? If you find the directory, what are the file types? It may be a data source rather than a flat file: go into the Control Panel>Administrative Tools, under the Data Source icon has denplan set up any data sources?
Post back what you find.