Union joins two sets together, returning the members of each set, optionally retaining or dropping duplicates (default is to drop).

This creates a single list of the top 5 and worst 5 products.
{UNION(
TOPCOUNT( {TM1FILTERBYLEVEL( {TM1SUBSETALL( [Product] )}, 0)}, 5, [Test].([Posting Measures].[Amount]) ),
BOTTOMCOUNT( {TM1FILTERBYLEVEL( {TM1SUBSETALL( [Product] )}, 0)}, 5, [Test].([Posting Measures].[Amount]) )
)}

To create a list of products that sold something both in this cube and in another (e.g. last year and this):
{UNION(
FILTER( {TM1FILTERBYLEVEL( {TM1SUBSETALL( [Product] )}, 0)}, [Test].([Posting Measures].[Amount]) > 0 ) ,
FILTER( {TM1FILTERBYLEVEL( {TM1SUBSETALL( [Product] )}, 0)}, [Test3].([Posting Measures].[Amount], [Entity].[All Entities]) > 0 )
) }