Could you read from the grand staff before you took the 10-year break?
if the answer is yes, it's just possible you have developed a problem with your vision that is restricting your field of view, but isn't severe enough to limit other visual activities. Getting an eye test might help.
If you have never been able to read the grand staff, I would guess the problem is partly psychological. You have discovered some "coping strategies" that partially work, and you have to find a way to "break" those habits, but that means accepting worse reading ability in the short term. A similar situation would be trying to learn to touch-type if you can already type pretty fast and accurately with two fingers.
Part of the problem may be that you can read treble clef easier than bass. The answer to that is the obvious one - practice reading the bass clef on its own.
One suggestion would be to find fairly simple music where you can't "fake" the reading, for example Bach's two-part inventions. There is only a single line of notes for each hand, but both hands are equal difficulty and there is no "solo and accompaniment" or "chord progressions" to fall back on.
Another suggestion would be make the problem harder, so you can't avoid it. Get some easy-to-read scores of string quartets (e.g. Haydn or Mozart) and sight-read the first violin and cello parts together, ignoring the two staves in between. You can download as much sight-reading material as you want free from http://imslp.org/.
Five of ten minutes "hard work", where your only objective is to fix this reading problem, will probably give more benefit than trying to combine improving your reading with your other practising and playing time.
From the answer it should be obvious I'm a "classical" musician, but the ideas should read across to other musical styles.