Assets in exchange traded funds will soon cross $2 trillion — a significant threshold for this relatively young investment vehicle. That’s no mean accomplishment. It took almost 18 years for ETFs to reach $1 trillion in assets, but only about four more years to double to $2 trillion, according to ETF.com. The first ETF — the $199.2 billion SPDR S&P 500 (SPY) — debuted just over 20 years ago in 1993.