How can I work on two apps at once?

I need to work on a text document and the slide presentation package at the same time, but can only open ONE of them. If I open the second, the first one closes!

One instance of LibreOffic can handle many documents at the same time. they may be of any type(s).

Concerning what you call “app”:
What did you actually do to “open ONE of them” and then to “open the second”?
In what way did you find out that “the first one closed”?

Your OperatingSystèm?
On recent Win the icons per document are “stapled” per document type in the taskbar.

I open (e.g.) the Writer app first, but then I can see no way to open Impress. If I re-open the entire LibreOffice home page, only the Writer appears. If I try to open Impress, Writer vanishes! Windows 10, fully updated. Will keep trying and tell you if I find a solution. Thanks. R. Beaton, York

Start or Open your Writer document, then start or open your Impress document.

In Windows you’ll see a text and a presentation icon in your taskbar

I forgot, you can either choose the program to work in in the taskbar or you can use the minimize/maximize/size buttons. With the size buttons you can put both app’s on on screen side-by-side if your screen is wide enough to be productive this way.

How do you open the “Writer app”?
Where did you look for a way to “open Impress”?
LibreOffice is one application with different modules and allowing to create(new) / open /edit documents of different kinds independent of the question on what kind of document you are currently working.

Use the appropriate one of the first four items of the ‘File’ menu.

If you have more than one LibO links on the desktop or whereever, they don’t address actually different applications, but will always stert LibteOffice itself. Just as an additional service they will also create a new file of the respective kind from the default template. I would suggest jou remove all the LibO related links except the one addressing LibreOffice itself. The additional creation of a new file is very simple when LibO is running (See ‘File’ menu as mentioned above).
If you prefer the distinguishing links, and you already are working on your new text document e.g, you always can minimize it (and probably additional open documents) to get full access to your desktop again - and to the links it contains.