One battery discharges faster than the other in my solar power system
I use the batteries as much as possible before my inveters beeps.


You are running the batteries flat. This is bad because the cells won't all have exactly the same capacity, so some will run out before others and may eventually get damaged. You should never let the batteries run right down, but make sure they get a full charge as often as possible so that the cells stay equalized.  

In full sunlight your panels can deliver ~20A of charging current, but to fully charge a 100Ah battery you need at least 5 hours at 20A (the 'bulk' phase), plus another 10-15 hours at reduced current (the 'absorption' phase). This could take several days.

One of your batteries may now be permanently damaged from over-discharge. To find out if it is recoverable, charge it by itself (with a 12V charger) until it can take no more charge (this may take 20 hours or more depending on charging current). Note how much charge it took. Then charge the other battery by itself until it too is fully charged. 

After charging the batteries individually they are now equalized at full charge. the amount you can take out is determined by the weakest (lowest capacity) battery. You should take out no more than 80% of the weakest battery's capacity, then put back in more than you took out. That way the batteries should always return to full charge and stay equalized.