There is no ,s, switch for $readini .. at least, not according to the help file.
Code:

on *@:OP:#:{
  if (!$readini(op.ini,#,$opnick)) {
    writeini op.ini # $opnick $nick 
    aop $opnick # 3
  }
}

However, with your writeini format, your .ini file will look like this:

[#channel]
nickThatGotOpped=nickThatOppedThem

Therefore, I don't see any reason why another channel couldn't have the same opnick managed separately.

[#channel1]
Hammer=Othello
ParaBrat=Hammer

[#channel2]
Hammer=Othello