You may use dex. It is a Python program meant to "generate and execute DesktopEntry files of the Application type" (quoting its manual).
Just put in your custom desktop entry a key along the lines of:
Exec=dex /path/to/application1.desktop /path/to/application2.desktop ...
(You may then be required to run
update-desktop-database from desktop-file-utils, or some equivalent utility provided by your desktop environment, to have your custom desktop entry show up in menus and other places).
Note that, as far as I can tell, while your desktop environment's application launcher may allow you to open a file or URL by typing
dex doesn't allow for arguments to be passed to desktop entries from the command line.
As an alternative to
dex, you may be able to use some command line utility provided by your desktop environment (e.g.
gtk-launch) as discussed, for instance, in this answer on U&L and some of the answers to this question on Ask Ubuntu.