One on Gtalk and one on Gmail.com while you're logged in via inbuilt web messenger!! else there are ebuddy and Meebo.com as well, and if you want to do it with Gtalk only then Check this out!!
I will tell you a dumb idea if you mean opening gmail messenger with two different IDs. One simple way is to open one ID with your Gtalk and the other one with your gmail account in the browser (chat enabled/standard view). If you mean to say that two gmail messengers with the same account. That I don't know
It is possible to open multiple gtalk messenger simultaneously by changing command line parameter. This is quite easy. Read the simple steps given here http://www.techfuzon.com/2008/07/multiple-login-for-google-talk.html There is no need to open different browsers.
In firefox you have got the option of having multiple profiles and each browser in a profile can have multiple accounts
Open one in google talk and other in the browser. In this you could run twice acc. OR You can open two different browsers and open gmail for chatting.