I have read many places that no two gemstones should be used. Use one only at a time AND
All combined gemstones ring should not be used.
I am of a opinion that 2 gemstones can be used if they are basis of supportive planets for you as per your Kundali. and also Nav-graha stone ring can be used.
In my opinion the gemstones which has backing of identical planet or co-operative planets to each other can be used in order to accelerate the effective power.
Another major thing to remember that when you use more than 2 gemstones you should be more enough able to judge what is effect on your day to day life. If anything negativeness observed then discontinue use of more than one gemstones. Wait for 9 days and again wear it then observe what has happened if same thing continued then you should be away from using it. This is more reliable tester.
We should keep away these stones while drinking, latrin or during sex and even ladies should not use STONES when they have periods. These stones should be charged once in a 9 months by washing with milk and keep them in starting sunrays.
Every gemstone has life to use if they are uncharged.
You can safely upto 3 years un-charged. If you charged it regularly then no pbroblem, also after 3 year you judge that what was happening with you immediatly after starting use of these gemstones is continued after 3 years also, in that case your charging is o.k. In case of negative feeling you can check with use- keep way-use and feel for 9 continuous days and decide becuase you are the best judge than any Jyotishi………………