A blended ROAS number — "our overall ROAS this month is 2.6×" — is a useful executive summary. It is a harmful decision input. Once you blend Meta's 28-day-click / 1-day-view window together with Google's last-click window into one spend-weighted figure, you have hidden the structural difference between the two channels behind a smooth number that nobody on your team can audit back to raw spend.
Spendquill's default is to render both per-channel numbers, in the same row, with each window labeled clearly. If a stakeholder asks "what is our ROAS?", the answer is a small table, not a single number. The answer carries the dissenting numbers with it.
The blended row is still in the report — spend-weighted, deduplicated, no model-shopping — because it is a useful summary line for the CFO. It just is not the headline. The headline has to be something a person on the team can defend line by line.