In Pokemon fire red how do you get the second gem for celio?

You don't get a second gem for Celio in Pokemon FireRed.

Celio only requires one gem, the "Heart Scale", to evolve his Shelgon into Salamence. This gem can be found in the "Victory Road" area, specifically in a small cave located in the western part of the route.

It's worth noting that Celio doesn't actually give you the Heart Scale himself. You need to go to Victory Road and find it yourself.