If I have pheons on both servers, when the servers merge will they both get combined (added together) after the merge or will they only take the one that has the highest number of pheons (resulting in me losing pheons from my other server)?
Yes, as @Mushrooms mentioned, currency you have on both servers that are going to be merged will be combined, you can read more details about the server merges here:
Crystal, Gold, Silver, Pheons, Phoenix Plumes, Pirate Coins, Coin of Courage, Providence Stones, Gienah’s Coin, Septrum’s Coin, Arcturus’s Coin, Ancient Coin, and Sun Coin will all be combined across servers to grant the player the sum of their values.
Hope this information helps, have a good day in Arkesia!