To expand on this, each plugin will receive host managed buffers that they're requested to fill and the input they're expected to process. If they don't do that in time for the host to mix the buffers and deliver the mixed buffer to the audio driver, it simply won't. Nowhere do the plugins directly interact through this process.
If your plugins are crashing because of an underrun you have a much more serious problem than underruns. Then you have plugins writing to or reading from memory that wasn't either handed to them by the host or allocated by themselves. That bad code running in your process can cause it to crash is an orthogonal problem to buffer underruns causing skips or stuttering in audio.
If your plugins are crashing because of an underrun you have a much more serious problem than underruns. Then you have plugins writing to or reading from memory that wasn't either handed to them by the host or allocated by themselves. That bad code running in your process can cause it to crash is an orthogonal problem to buffer underruns causing skips or stuttering in audio.