The composite meter is a bit misleading, isn't it.

If the entire piece alternates between 3/4, 5/4, then create a blank bar of 3/4 and a blank bar of 5/4. Change the display of each to the composite meter, then copy the two measures throughout the piece. That should do it.

EDIT: If you've already put the music in, you can always edit the filter to just the time signature and copy two measures you've changed for display and then "paste multiple..." across the document. You'll have to rebar the music from the utilities drop-down menu, though.