Tony, what you say about the added or diminished relative scopes of
the knights and bishops in double-board variants is true, just as
it is in larger variants to begin with (the knight is an extremely
weak piece in 10 by 10 variants) but the beauty of a game like my
Doublechess variant which I invented is that the knights still 
have their roles to play. Like I said before, pieces on each half
of the board tend to engage each other at the same rate they do in
regular chess. Pawns challenge each other, knights move up to the
third (or sixth rank, for black) rank to attack enemy pawns,
files open up for rooks and queens, diagonals open up for bishops
and queens. 

I think one point that needs to be made here is that in
Full Double Chess, stronger pieces are used, and that's fine,
if you are a player who likes new fangled pieces that can do neat
little tricks and jump through hoops. My Doublechess is more traditional,
uses only orthodox pieces and has the look and feel of traditional 
regular chess. So whether a game like my Doublechess or the new
Full Double Chess appeals to someone is going to be a matter of personal
taste, I guess. 

p.s. I would still like to encourage people to add comments below to
my Doublechess variant, for which I began a discussion.