If I have 2 brawling gloves eg: cooking with 750 charges, so I used one of them halfway like lets say 500 charges left. Then I put it back in bank with another gloves already in bank. Will the total charges left is 1250 or 1000?

You have to fully use one before the others charges will be restored to 750. 

To explain this better:
The charges from brawler gloves are a value that is stored in your account, rather than the gloves themselves. This is why they are stackable in your bank in emps, but aren't in RS. Let's say you have 500 charges left on a hunter brawler, but have two of them. Should you run out of those charges, your charges will be restored and it will consume one pair of brawlers, leaving you with one pair of hunter brawlers.

To directly answer your question: 1250

Thanks, got the answer.