I mean something like Kerbal Space Program style - payload between 2 parallel rockets e.g.
Also, space shuttle configuration comes to mind where we have the large fuel tank between 2 solid boosters: space shuttle rocket
I imagine such a payload between 2 Falcon 9 or Proton M rockets e.g.
I know this will have issues judging on Falcon heavy where they combined 3 boosters that are separately tested and needed years of tests to make it work, but is it feasible and has anyone considered something like that?
edit: I know the Kerbal example won't work in real life, my point is instead of developing new super heavy lifting vehicles can't we just use few of our reliable rockets to go to the Moon or Mars.