Hi I was that opponent and looking at it from a MTG point of view the wording is just fine and it's working like intended.
"While Martyr Golem
is deployed, all damage that would be dealt to a Character you control is redirected to Martyr Golem
instead." Martyr Golem
can be read here as "this card".
It would make no sense to be punished and receive double damage when you have two of them out (which you were suggesting), neither will the damage be split between them.
A simular card in MTG is "Empyrial Archangel" for example.
Here's some ruling that goes with it which can be applied in IW as well:If you have two Empyrial Archangels in play you would only choose one of them for each instance of damage occurring, and you get to decide which one (419.1a; 419.6c; 419.9a).
The player receiving damage can actually chose which one takes the damage for each instance of damage so it's even better there. IW doesn't use a stack like MTG does and therefor I assume they chose to go with the last Martyr Golem deployed receiving the damage as first since that's what happened when I played against you.
I understand you got worked up about it when it happened because it's not what you expected to happen. For things like these we actually need a comprehensive rule book.