Perhaps this isn't the most appropriate forum to post this question on, but I would very much welcome suggestions and comments.
Basically, I'm orchestrating a romantic piano concerto, starting from the current position of having an extant two piano score for the first movement, and a solo piano reduction for the other two. Sometimes it's quite clear which parts should be given to the piano (eg when the texture or writing is highly pianistic), and sometimes, because of the nature of such works re dialogue between orchestra and soloist, that a passage should be given solely to the orchestra. My question is, therefore, when a passage is being shared between orchestra and piano, to what extent is it acceptable or normal for the orchestra to double the piano, even if in different registers, or should different supporting, even perhaps separate contrapuntal, parts be deployed in the orchestra. I understand that colour and timbre are considerations here. The only previous advice I've received, and I think it's a fair comment, was that the two piano score didn't add much to the solo version and was a bit perfunctory.
To give examples of my thinking, here is my draft commentary on how I plan to deal with the start of the third movement (apologies to anyone who has already seen it!)
14.00 (bar 1): obviously a solo piano introduction
14.18 (11): orchestral, strings plus woodwind and possibly timpani
14.30 (15): bassoons plus piano
14.41 (19): piano mini cadenza - I've evidently got this notationally wrong in terms of note duration
14.45 (23): probably both piano and orchestra, but I'm very unclear about how I would lay out this passage
15.00 (31): piano, string harmony for colour
15.03 (35): Dies Irae in low woodwind and possibly low strings; the octave lh figure is obviously piano, but it seems a bit of a tautology if the rh is just copying the ww+strings.
15.13 (39): currently undecided as to instrumentation, but maybe orchestra seeing as the next passage at 15.49 is clearly pianistic.
I'd really appreciate any thoughts, this is a completely new departure for me and when it comes to composition I am completely self-taught.