You are only moving massive amounts of money if you are moving massive amounts of money. Having massive amounts of money invested in ETF's doesn't mean you are executing massive trades if you are holding endowments and your only trades occur when you invest new gifts, withdraw a constant income stream, and rebalance your portfolio mix on a regular basis.
The way these index ETFs work though is that broker dealers can trade a basket of securities matching the index for a share of the ETF (and vice versa). Because of this price mismatches get fixed very quickly. S&P announces also changes ahead of time so while there is initial price movement it's not all instantaneous.
The main planning I could see overlap is executing large trades since they're both moving massive amounts of money.