Multiple apps. Or the m.twitter.com as far as I know no app supports multiple concurrent sessions.
Yatica, socialscope, twitterberry are a few more you could check out, none are that "light-weight" size wise though
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Q10! <- Love the 9000!! It's everything I wanted (minus enough ON BOARD memory)!!
<-9700 ---! more memory! small screen&keypad :( ->