Yes, both legs must be switched. The reflection of the if the unterminated filter will be across the
one in use. At a bare minimum you have to test to insure it will work but generally its going to
Its not a shared ground. Its the shared hot! What does a 40M filter look like at 20M?
What does a 20M filter look like at 40? Hint the 20M will pass 40M signals (and possibly harmonics),
But the 40M will not pass any 20M signals. So a filter that does not pass 40M across a 20M filter
has to have bad effects.
You need two sets of contacts.