You have probably activated the waitlist under the crossfader by clicking on "W". If the fader can now be seen left-aligned with you, simply click on "P-List" below to reactivate both playlists.
If you have also closed the fader with the X in the top right, you will see a small triangle in the middle on the left. There you can open the fader again.
The manual says the following:
>> 3.3.2. Waitlist, single list mode
In addition to the option of providing a separate playlist for each player, there is another display variant, the waitlist. In this mode, both playlists are combined into a single list. About "W"
and "P-List" in the crossfader toggles between dual list display (playlist mode) and single list display (waitlist mode).
When switching from dual to single list, the title lists, starting with the first title from playlist A, are merged in a comb shape. When switching back to the dual list display, the titles are alternately divided between the two players, the odd titles in playlist A, the even titles in playlist B. <<
The new BPM Studio Profi 5 version is largely based on BPM Studio 4. You can also use the manual for BPM Studio 4 for BPM Studio Profi 5. You can find this as a download at https://www.alcatech.de/downloads/.