 Bloomberg -- Profits at TD Ameritrade Holding Corp., and its buyer Charles Schwab Corp., have been goosed by a quantum leap in retail trading volume that may be set to fade, according to Citi.Fiscal third-quarter earnings at TD Ameritrade beat analyst William Katzs expectations. But now the stock faces a pivot point around the sustainability of surging retail engagement thats been catalyzed by the Covid-19 backdrop, according to Katz.If this is indeed the new normal, our stand-alone and pro-forma estimates could be sharply too low, Katz wrote in a note. But, history would suggest such engagement may not be fully sustainable, leaving us a bit in the show-us camp.Katz revised his estimates to assume greater trading activity but also to account for an easing pace of growth and net interest income contribution. Additionally, he lifted his price target for TD Ameritrade to 42 from 41, and for Schwab to 38.50 from 38. For both, he said residual upside is a bit shy to step into the stocks, particularly given some potential tax rate risk emerging with the U.S. election and whether investors want to chase the retail engagement cycle.TD Ameritrade fell as much as 1.4 in Wednesday morning trading; Schwab slipped as much as 1.6. Both have underperformed so far this year, with TD Ameritrade down 24 and Schwab falling 26, versus a 0.8 gain for the SP 500.On Tuesday, Toronto-Dominion Bank said its stake in TD Ameritrade will help cushion the earnings blow from the coronavirus pandemic as TD Ameritrade posted third-quarter profit that beat the highest analyst estimate, thanks to the boom in retail trading seen across electronic brokerages.For more articles like this, please visit us at bloomberg.comSubscribe now to stay ahead with the most trusted business news source.2020 Bloomberg L.P. 