Hi @Gaurav_C,
SILVA has two sets of files, one for SSU and another for LSU. The SSU contains both the 16S & 18S sequences. Fortunately, we've made a great plugin to help you curate your own reference database: RESCRIPt. You work through this SILVA tutorial.
You can either make two different classifiers based on your amplicon region or simply use the premade full-length classifier available on the Data resources page. Check out this post for more details.
-Cheers!
-Mike