I think sightreading is a matter of recognizing patterns. It's impossible at the same time to read note by note, know which note you are playing, know the duration, and even more... to do it with two staves!
You have to learn to recognize patterns, I mean, if you see there is a dotted crotchet with a semi-crotchet, you already know how the rythm will be, then it's only a matter of pressing the correct key, for this you have to recognize the patterns of height in the stave, ecc...
When you deal with chords is the same, if you want to sight read a series of chords you don't read note by note, instead you read the whole chord, again a pattern.
I am not a teacher, I just learned to sightread alone, and after a long time sightreading I noticed that it is only a question of pattern recognition.
To give you an example, take a look at the Mozart Sonata in C, K.545, the first two measures... the LH makes broken chords. A C major chord in the first measure, starting in the middle C, so ready, in this measure you already know what to do with LH, you can play it wihout reading it and you have to deal with the easy melody in the RH. In this case you read blocks of four crotchets in the LH.
Then in the 2nd measure it's a bit harder, but you have to go on reading in blocks of four crotchets, the 2nd block is again a C major chord, so, forget it.... and so on.
My advice is to sightread both staves at the same time... if you find this difficult do it veeeeery slowly, and try to read blocks of notes, not note by note. It's useful to listen to pieces and at the same time read the sheet music. For your case it would be good if you do it with slow tempos, and be sure to know exactly which note or chord is being heard.
I hope this helps...
Good luck!