Sight-reading for piano is, of course, made harder by two lines, two clefs etc. If you can read each line well on its own, then the problem isn't identifying notes within each clef but using your eyes to scan both equally and looking ahead far enough and quick enough to keep going with some reliability.
This is a skill largely acquired by practising it! Sorry, no other way I know of but, that said, you can try sight-reading just a few bars at a time initially, maybe even only a couple. Sometimes looking at a long piece (even 8 bars can look long sometimes

) can be so daunting.
I'd start trying to read lots of small bits and pieces and then extend the length as you gain in confidence. Try to make sure you maintain as even a pulse throughout as you can and really encourage yourself forward to achieve that. It's so easy to play unrhythmically but an even tempo is, of course, one of the main ingredients of good sight-reading. Check out the key too!
Like much else to do with developing musical skill, there's no substitute for working at it.