(Reuters) - Two blank-check firms, led by veteran Wall Street dealmaker Michael Klein, collectively raised $1.68 billion in initial public offerings, their regulatory filings showed on Friday. Churchill Capital Corp VI raised $480 million after upsizing their offer from $400 million, while Churchill Capital Corp VII garnered $1.2 billion, raising from $1 billion. Blank-check firms are shell companies, also known as special purpose acquisition companies (SPACs). They use capital raised through a public offer to buy a private firm, typically within two years, in a deal that takes the private firm public. Investors are not told in advance which company the SPAC would buy. Klein, a former Citigroup banker, is one of the most prolific SPAC backers. In July last year, healthcare services firm MultiPlan agreed to go public through Churchill Capital Corp III, another SPAC led by Klein. SPACs offer an alternative route to public markets to companies seeking to avoid a traditional IPO. Their popularity surged last year, when some 248 SPACs raised around $83 billion through IPOs, more than the prior five years combined, according to data from SPAC Research. This year, digital ad firm Taboola, fuel-cell truck maker Hyzon and online learning platform Nerdy have announced blank-check deals to go public. Citigroup, JP Morgan, Goldman Sachs and BofA Securities were the underwriters for both the offerings.