Experts are rarely permitted to to listen to the proof. Witnesses are routinely excluded.
Expert testimony is not treated the same as witness testimony.
Experts are supposed to provide objective factual evidence within their area of expertise.
They are almost always allowed to hear the testimony of others, but especially the opposing sides experts, to aid the side them hired them in interpreting and formulating questions for the opposing experts.
I have testified as an expert witness a couple times as a professional engineer.