It works but it is ok to use both.
As according to Mobi guidelines we have to use only format for mobile.
If we use format like this, they can disable our domain or cancel the registation.
I think they should be fine as we are using both for mobile and for desktop ??????
one more interesting fact:
Just now tried mobi page with .html or .xhtml extension in follwing URL
http://emulator.mtld.mobi/emulator.php (Page given by .mobi to test the website)
When I tried the .html it worked fine but same time when tried .xhtml it didn't work
Any help ???????????