tl;dr: Usually yes, with an appropriate BIOS setting.
Since this is an old question, I suppose that you figured it out by now; but I'm going to answer just in case other folks wonder the same thing - because I just did extensive research on this :)
I understand why you have this question, because I recently asked myself the same thing after installing a discrete GPU in a B550 main board with a Ryzen 4750G processor. Installing the discrete GPU disabled the APU (it wasn't even showing up in PCI enumeration, for instance with lspci on Linux).
It turns out that there is a setting in the BIOS to control that. The setting has different names depending on the constructor of the main board, and the values aren't very explicit.
For instance, on my Gigabyte main board, the setting is called "Internal Graphics", and it can be set to Disabled/Auto/Forced; and it should be set to "Forced". (If it's set to "Auto", it will be disabled when installing a discrete GPU.) On other Gigabyte main boards, the "Forced" option will be called "Enabled" instead. On other boards the option might be called "IGPU multi monitor" or "IGFX Multi-Monitor".
There are some rare scenarios where that option might be missing from the BIOS, and then it might not be possible to keep the integrated GPU when installing a discrete GPU; but if you're wondering "will it work?" you can look for that option in your BIOS. If it's there, there is a solid chance that it will work!
Once both GPUs are enabled and detected, you can use either of them any way you like.
