2-engine VTOL aircraft

I remember hearing one of the reasons that the F-35 Lightning II and the Boeing X-32 had a single-engine design was the 'challenging' nature of having two engines perform in a VTOL system. This might have been refering to a Harrier-like vectored thrust system, but what exactly makes a 2-engine VTOL, besides for the need to balance out two engines worth of thrust and the increased weight, so impractical?

Specifically when it has been at least attempted, apparently with some success.

It would seem to me that a 2-engine VTOL design would be a place of compromise for Carier operations and STOL/VTOL needs.

basically, the above, addition mine

it depends on what you mena by twin engine.

Designs with sperate lift and forward thrust/cruise engines (i.e. the YaK-38, mirage IIIV etc) do have issues in that in level flight the li=ft engines are just need weight and they cna make thurst balancing very tricky.

Twin engine pegasus/BS.100 style installations have been proposed (notably the twin spey P.1154 proposals) but ultiamtely HS greatly prefered a single engine fit as it would be lighter, smaller and a lot easier to biuld, despite beign a bit slower. Out of all that it was the weight savigns that were the main drivign froce as with S/VTOLs even kg counts.

Basically its not so much that twin engine designs are impractical but that they tend not to have met the design requires for all the S/VTOL projects to date.