From my interpretation of your question I believe that according to what we know about what happens to people who eat two fruits. I will reference a few things as I go about answering your question.
The Revive-Revive Fruit that Brook possesses, also called the Yomi Yomi no Mi, is a Paramecia class that can bring the user back to life after death. But later in the series it is declared that the fruit has "further" capabilities and can enable several other things as referenced here. Brook uses these abilities in later episodes after the timeskip. As he found the "true nature of his fruit". Now whether or not that means he awakened his power or not is still up to speculation at this point, but I suspect we will see more at the next arc.
As shown here and here, there are a few special exceptions where a particular person can have more than one fruit, so it is theoretically possible for Brook to have multiple fruits.
Given that most folks can't have more than one Devil Fruit and that Brook's power IS NOT "used up", and referring here I will follow the fact from here:
Myth: When you eat 2 Devil Fruits, the second fruit will replace the first fruit's power.
Fact: Eiichiro Oda stated in one of his SBS that you cannot transfer the fruit's power to another person, if you take one bite of it the fruit becomes normal. This caused fans to think he meant if you eat the second Devil Fruit it will replace the first fruit but in Enies Lobby Arc, Lucci and Blueno said if you eat the second fruit after eating the first one, your body will explode and you will die.
Fact: An exception to the rule stated above is Marshall D. Teach, user of the Yami Yami no Mi fruit, who stole the power of the Gura Gura no Mi fruit through some currently unknown means not related to eating it.
I believe that if Brook ate a second fruit, his body would explode. Upon which the Revive Fruit which is there would try to activate returning his soul to his body, but due to no longer having a body via an explosion his soul would be trapped in the physical world but without a body to be in. Therefore he'd live but without a second ability, body or any method to do anything. He'd be a mass of floating "conscience". That is unless his fruit enables a possession technique of sorts which we have not seen yet. I hope I either answered your question or provided enough to read to help you arrive to a better one.
TLDR Brook would die, revive and not have a body or a second power.