Compares two US-listed stocks or index ETFs on identical dates with the same money, on daily closing prices adjusted for stock splits with dividends reinvested, the way SmartFin's Compare Two Stocks page does. Returns, for each, the value of the starting amount and its total and yearly return, the worst fall with its dates and the months back to the high before it (or that it hasn't recovered), and, with monthly_amount, the whole holding (with_monthly: purchases made, money put in, value, and total return on the money put in) with that amount added once for each 1st of a month after the start date and on or before the end date, on the trading day nearest that 1st. Use when someone asks how two stocks or funds did against each other, e.g. "apple vs microsoft stock over 10 years" or "voo vs qqq since 2015". Do not use for one stock (use smartfin_calculate_investment_return), for ways of buying one stock (use smartfin_compare_investment_strategies), for future returns, or to say which to buy: it states both figures and never picks one. Limits: today's S&P 500 companies and major index ETFs only, prices in US dollars; both start on the same trading day, and when one stock's history starts later, both start then and the summary says so.