I have written a subject and countersubject. Here is how I went about writing the countersubject:
1st iteration: Restricted to D, G, and A(answer is is D major), all quarter notes
2nd iteration: A bit more free, can use any note in the D major, G major, and A major triads, all quarter notes
3rd iteration: Completely free, complementary rhythm
4th iteration: Correcting it so that it is playable for a pianist like me who can reach a 9th at the most and for which the widest practical interval is an octave while making sure there are no parallel perfect consonances when I analyze the intervals in the melodies.
But with a second countersubject, it is like it is easier but at the same time more difficult. Easier because you already know the harmony and what notes would be consonant in that harmony, harder to avoid parallel perfect consonances, have an independent melody, and have the preexisting harmony, all at the same time. But I am thinking of having a second countersubject and possibly a third for my 4 voice fugue in G major.
I know I could just leave it with the first countersubject and go straight to free counterpoint in the soprano once the tenor line plays the subject. But I tried that with my C minor fugue and it was hard. It got to the point where I was like "I wish Bach was here" and said R.I.P to my fugue in C minor.
So how do I go about writing a 3rd or 4th independent melody to use as a countersubject while avoiding parallel perfect consonances? Same way I did it with the first countersubject? And also is it okay if I have for example parallel octaves in the outer voices once I have 3 or 4 voices, as long as I don't do it too much and never have parallel perfect consonances in sections with only 2 voices?