In reply to J1234:
It's not unusual - I have a bike with dura-ace on it and it doesn't like operating big/big or little/little. It also has midway positions on the front shifter because the tolerances are so close - it effectively has 'big ring left' and 'big ring right' positions. Having said that, it shifts quickly up and down with a swift click, until it gets out of tolerance at the front and the front derailleur needs a nudge.
Shimano advise against big/big or little/little. SRAM say it doesn't matter and I forget Campagnolo's position.
Either way, with a widish chain (i'm guessing 8 speed for your bike) and three rings, one extreme to the other isn't going to be good. The middle ring should cope OK though.
As a guide - big ring to smallest sprocket - chain should just clear the front derailleur and rear DR should be in line with the smallest sprocket. IF you pull the cables by hand, neither DR should move further out.
Little ring to biggest sprocket - chain should just clear front DR. Rear DR should be in line with largest sprocket. Tension should be out of the cable but not loose. The DRs will have two screws - one marked H and one L. The L one adjusts the position of the rear DR in relation to the largest sprocket (Low gear) and H the position in relation to the smallest sprocket (High gear). Front DR is opposite way round. Screwing the screw in extends the protrusion into the mechanism, and restricts the movement - It's easy to feel with a finger. Adjust the cable with the barrel adjuster (see a website - shimano, park tools or sheldon brown)
Adjusting is easier with a bike stand, or hang the bike by the saddle on a broom handle resting on two wheelie bins
Post edited at 11:58