Marica, your question is too broad to definitively answer. Where is your primary listening post? What type of speakers? What type of bass response are you seeking? These are just a few questions that must be addressed.
Actually I need sound as evenly distributed all over the room as possible. I'm aware that this leeds to many compromises, but those are my priorities. I need just a simple rule for placing 2 speakers in such a room.
Put "Speaker Placement" into a search engine and you will get several set up possibilities. These are all meant to get the best performance out of the system. Even distribution depends more on the loudspeaker design than it does placement alone. Most rooms are fairly reflective when it comes to sound and too much distribution is often what you have to control.
Marica, it's usually on the wall with the longest dimension. That said we usually describe the location of the speakers in relation to your main listening position. The relationship is such that the speakers should be set apart by a distance less than or equal to the distance of each speaker to your listening position so you form an isosceles triangle (or an equilateral triangle in the case the distances are equal).
Once this triangle is maintained you will get the best reproduction for your favourite listening position. However, room reflections affect how the sound is propagated in the rest of the room. If the speakers are placed on the longer wall, then they have less reflections from side walls interfering with the sound, and the distance travelled by all the sound is lower, giving a more uniform soundscape generally.