You need to have a blank line in between trackers for different sites.
ht tp://tracker1.site1.com/announce
ht tp://tracker2.site1.com/announce
ht tp://tracker1.site2.com/announce
ht tp://tracker2.site2.com/announce
ht tp://tracker.site3.com/announce
ht tp://tracker.site4.com/announce
In this example tixati will use the first working tracker for site1 and site2, but not both, it will try the 2nd if it cant connect to the 1st, it will also use the trackers for site 3 and 4.
You will notice on the left side a group number as well, again in the above example, site1's trackers are group 1, site 2 is 2 and etc.