After spending Tuesday trying to put out one merger-related fire, America Online ( AOL) has to hope another one doesn't flare up Wednesday morning on Capitol Hill. AOL, which is in the midst of gathering regulatory approvals for its planned acquisition of Time Warner ( TWX), was on the defensive Tuesday after The Wall Street Journal suggested that AOL was trying to create a worldwide instant messaging system by linking its AOL Instant Messenger and ICQ services. That has raised the ire of AOL competitors like Yahoo! ( YHOO) and CMGI ( CMGI) subsidiary Tribal Voice, which say that AOL, as a condition of the Time Warner deal, should be pushed into making its systems for sending short text messages compatible with those of its rivals. AOL, which operates AIM and ICQ as separate services and doesn't enable users of one to send instant messages to the other, is estimated to command at least 90% of the worldwide usage of instant messaging -- a technology that hasn't generated much revenue for anyone yet, but which some boosters say could be as important a medium as the World Wide Web and email are today. The Journal article, the accuracy of which AOL disputes, comes one day before AOL Chairman Steve Case and Time Warner Chairman Gerald Levin are slated to testify on the future of interactive television at a hearing by the House Telecommunications Subcommittee. As is the case with instant messaging, interactive TV is a minuscule moneymaker now, but has vast commercial possibilities -- and could be unfairly dominated by a merged AOL Time Warner, say other companies.