When it is fedrinka. Because they are strong in an image of the double. Roger and Stan being gold medalists of the Swiss team.
However, there is more play of the game of singles than a double now. And they practice well together. They may not play the London Olympics in a double. etc.
and it's better as Federinka than Fedrinka because FEDERER and WAWRINKA it is a more smooth transition as it hinges nicely on the "R" and each player gets exactly 5 letters from their own name into the combined name so it's perfectly equal.