VSTs simultaneously reading/writing audio buffers in memory or files on disk

First of all, is it possible for a VST or anything else written in JUCE to leave an audio file open on disk to be both written to and read from simultaneously? If so, could it be done by multiple instances of a VST?

What about multiple instances of a VST reading/writing to the same audio buffer in memory?

I should clarify, I didn’t mean VST specifically; it could be any audio plug-in (.dll).