i have the same issue with Joomsef ,expecting using the language filtering redirection on 2 differents extension domainname
I wanna do a multilanguage website
Cant find a solution when enabling Joomla Sef filter language i receive error 404 as you

How did you solved this issue ?
I have a non paid joomsef but before buy it for this feature i wanna be sure that works

With the joomsef non paid i cant set .htaccess it maybe that but not sure

All info in the doc i got seems coming from old version of Joomsef or Joomla
I m using Joomla 2.5.4 and Joomsef 4.2.6 last update

Even with the Joomla Multilangue module and plu gin off it stills not working even if i change template
All caches are purge & clear

This issues come only when enabling Joomsef Language Filter
thanks