I have created table-valued function which accepts two parameters, first is string with IDs, and second is delimiter in string.
CREATE FUNCTION [dbo].[Split](@String nvarchar(4000), @Delimiter char(1))       
returns @temptable TABLE (items nvarchar(4000))       
as       
begin       
    declare @idx int       
    declare @slice nvarchar(4000)       
    select @idx = 1       
        if len(@String)<1 or @String is null  return       
    while @idx!= 0       
    begin       
        set @idx = charindex(@Delimiter,@String)       
        if @idx!=0       
            set @slice = left(@String,@idx - 1)       
        else       
            set @slice = @String       
        if(len(@slice)>0)  
            insert into @temptable(Items) values(@slice)       
        set @String = right(@String,len(@String) - @idx)       
        if len(@String) = 0 break       
    end   
return       
end  
After creating function, just use UNION set operator on this way:
EDITED
WITH ListCTE AS 
(
select items from dbo.split('400,600,150,850,160,250', ',')
union
select items from dbo.split('600,150,900', ',')
)
SELECT TOP 1
   MemberList = substring((SELECT ( ', ' + items )
                           FROM ListCTE t2
                           ORDER BY 
                              items
                           FOR XML PATH( '' )
                          ), 3, 1000 )FROM ListCTE t1
With UNION you will automatically get distinct values from both strings, so you don't need to use DISTINCT clause