IDF webers are a downdraft carburettor, while DCOE's are a side draft. So you can't use the DCOE manifold. I've never seen twin IDF's on an
A series motor, but a manifold could be made. it just depends on how good you fabrication skills were. The best way to get this done would be find a Pattern Maker, who would be able to make a mould & get it cast for you. It'd be expensive, being a one off. You could always use an old alloy head as a source of the alloy for the manifold.