The Plus! log viewer can already show Messenger's XML logs.
What do you exactly mean by syncronizing? Because with syncronizing I understand making both folders having the exact same files. And what is the point in that? To have two files of every log?
Simply point your log folder in Messenger to the same directory you use in Plus! if you want all the logs in the same place. Otherwise, use one or the other. Again, I dunno what you exactly mean so... but what I understand from it, it seems very pointless.
What you might mean, and be a bit more usefull is a converter to convert Messenger logs to Plus! logs. But that is something completely different to what you've just requested. And that simply requires an altered XSL file which you put in your Messenger log directory, no external program or internal change required.