Merge and get unique data store in one variable

I have two variables like below.
@SiteNames1='a,b,c,d'
@SiteNames2='a,b,x,y,z'
declare @SiteNamesNew varchar(1000)
I need to store values in another variable @SiteNamesNew with distinct values.
Result set would be- a,b,c,d,x,y,z
Please help on this.