I am not sure, whether its there or not.

Is there any app to connect to another phone via bluetooth and then use both the phones as two speakers to play some MP3 song.

Both the speakers will be acting acting as stereo (Left and Right) and will be playing the song with louder sound.

If its still not there. I think it must be released soon.
Any members, if can do, please try requesting symbian programmers and authors to release such an app.

Your idea is very good. I am sure if there is any app like that, it will be very useful.
I will contact some authors for that.

woah... thats a very innovative idea.. but, with the quality of nokia speakers i doubt there will be any such app coming out soon.

and wont it look weird if u take two N90s and put them near your ears... lol...

No, I think bacman is right.
When you are with some friend, you can listen to the same song as he is, together, or if you can connect it to even more phones, it will be fun.

The main advantage would be, you will have louder sound to hear.
Or while going in a bus, or car, you can keep your phones on both sides of your seat, to hear stereo music, with louder sound.

Me and my mates used to do this, in school bus, but we do manually, play (the song together). Also we can make echo-effect of one phone with the other one. It sounds good (totally 3D). Try yourself!