AS Monaco's Falcao, who also played for Manchester United and Chelsea, is suspected of failing to correctly declare 5.6 million euros ($6.1 million) of income earned from image rights between 2012 and 2013 while he was at Atletico Madrid, Madrid's regional prosecutor said in a statement.
Coentrao, who plays for Real Madrid, is accused of hiding nearly 1.3 million euros in income earned from his image rights between 2012 and 2014, it added.
The two players are suspected of using a web of shell companies in the British Virgin Islands, Ireland, Colombia and Panama to avoid taxes on the income from their image rights.
A Spanish court last year found Barcelona's Argentine striker Lionel Messi and his father Jorge guilty of three counts of tax fraud resulting in a suspended 21-month prison sentence for both men.
It ruled Messi had defrauded the Spanish tax office of almost 4.2 million euros from 2007-09 by using a web of shell companies to avoid paying tax on income from his image rights.
Under Spanish law any sentence under two years for a non-violent crime rarely requires a defendant without a previous conviction to serve jail time.