The panes are fixed by the brief; what is open is how they share the width, and what happens when the SME collapses one. Pick one and I will build it out — the screen above is drawn in 2a.
Evidence takes the slack. The browser is a reference pane, wide enough to confirm context but not to work in. Best for automated triage, where the screenshot answers the question.
Inverts it for live sessions and manual findings, where the SME is operating the page rather than reading a capture. Evidence narrows to a thumbnail strip.
Both viewers get the full remaining width, stacked. Screenshots and the live page are the same shape, so they compare directly — at the cost of half the vertical measure each.