To get all the notes of Symphony no. 40 by Mozart on the piano you need at least 7 pianos if not more because of 2 bass clef instruments and/or 2 treble clef instruments and/or 2 playing the same note.
Here is how I would assign them;
Primo Piano 1: flute part
Secendo Piano 1: Oboe 1
Primo Piano 2: oboe 2
Secendo Piano 2: Bb clarinet 1(meaning you have to transpose everything down a half step since Bb is C on the Bb clarinet)
Primo piano 3: Bb clarinet 2:
Secendo piano 3: Bassoon 1
Primo piano 4: Bassoon 2
Secendo piano 4: Horn 1(Bb, Eb, or G horn meaning you have to transpose everything down by the difference between the note that is C on the original instrument and that same place of the staff on the piano)
Primo piano 5: Horn 2(Eb or G)
Secendo piano 5: 1st violins
Primo piano 6: 2nd violins
Secondo piano 6: Violas
Primo piano 7: Cellos
Secendo Piano 7: Contrabasses
Now the way I would let them play is the person playing the flute part first practice on his own and let the others practice on their own. Than I would conduct a gradually increasing number of players until I have the whole 7 pianos. Would I actually need a bass instrument and a treble instrument per piano? This would be like right hand only so could I really have 4 instruments per piano 2 treble and 2 bass and thus half the number of pianos I need to 4 pianos? If so that would be awesome but than again they will have trouble with syncronizing 2 instruments for even just 1 player, more difficult than syncronizing left and right hands of a piano sonata that is because it would almost be like there is a polyrhythm even though everything is 2/2 for 1st movement, 6/8 for 2nd movement, 3/4 for 3rd movement, and 2/2 for 4th movement.
Would I need to start with symphony no. 1 in Eb that is written for 2 oboes, 2 horns, and a string quartet thus 2 pianos and than go all the way to symphony no. 41 in C for flute, 2 oboes, 2 bassoons, 2 C horns, 2 C trumpets, Timpani in C and G and a String quartet adding up to 15 instruments for 4 pianos to really get the best performance of Mozart's Symphony no. 40?