I guess your main concern in this is not whether this is a proper fugue or not (since you mention you are happy with it as a piece rather than fugue). Usually there are two approaches to writing double fugues: 1) The two parallel fugue subjects come in simultaneously from the very beginning or 2) The second subject gets its own fugue after the first and after that they are (possibly) combined into one double fugue. I am not sure which approach you were going for since in the beginning there is a little time before the second subject comes in, but on the dominant entry (meas. 16) the second subject does play simultaneously. Maybe you just weren't concerned with writing a strict double fugue. The entry in the cello in meas. 32 is also elongated a little bit in comparison to the original subject. There also doesn't seem to be a very clear delineation between your subject entries and their developmental episodes. But that's all beside the point of whether the music is enjoyable or not (which it is). The only thing that bothers me on that front is your frequent middle entries in the same keys (meas. 64, 79, 130, 134 are all in Bb minor, and meas. 172, 203, are both in C minor (although interjected with some G minor entries)). Although there is a welcome respite from the harmonic repetition when you have the middle entries in F minor (153 and 159). I do think that episodes should really stand out from the rest of the piece in having a developmental/sequential character which is missing here (meas. 146 is a good exception though). Also, the final entries lack the feeling that they are closing the fugue - maybe a tonic pedal would help. Right now it feels like the fugue just stops. Those are my thoughts ... hopefully they help! Thanks for the music.