"Ed Kyle" wrote in
oups.com:

Makes me wonder - could a launcher be created
using only two existing core first stages
side-by-side, rather than spending bucks to
develop a new fat stage? A double-core would
double the liftoff thrust.

It's hard to tell for certain, but that's what Boeing
is apparently thinking of for the 'Delta V' widebody
first stage.

Using existing stages in series with a large second
stage may lead to a very long stack; a paralleled
configuration looks more rigid and may present a less
complex structural and control challenge in atmospheric flight.
The interstage might be more complex and heavier.

At any rate, I can't think of an operational example
of just two paralleled first stages with all upper
stages on top; must be a reason for it. Otherwise,
it seems like a logical approach to heavier launch
vehicles.

One could say that Saturn I and IB were paralleled
Thor? stages, I suppose. Or perhaps Proton was
a paralleled derivative of some ballistic missle
launcher?

--Damon