Is it possible to play 2 sounds simultaneously?
For example: one via HDMI (audio from a video) and another one (custom audio, wav, or whatever) via analog output at the same time?
The Raspberry Pi can only use one output. This is by hardware design and is to do with DMA (Direct Memory Access)
You can plug in a USB audio device then you can use two output sounds at the same time.
The next best thing could be to play a sound out on HDMI, switch to Analog and play a sound there. But not at the same time and switching could make strange noises on both sources.
It's possible (at least on Raspberry Pi 3). I have Kodi playing audio through HDMI and MPD playing audio through the analogue output. They can do so simultaneously. See my answer to a similar question.
In short, send one sound to card 0 device 0 (analogue) and another to card 0 device 1 (HDMI).