The extra speed - and often more complicated systems - makes single pilot light twin IFR (or IMC, no great difference to my mind) a very high workload. It's often said that SP IFR is one of the highest workloads in aviation.
Still, as long as they both keep turning it's not much more difficult than doing the same in a high performance single eg C210 or A36. It's when they don't both keep turning that the twin becomes a very real problem that **must** have a trained & recent pilot at the controls.
If you value your life & those of your passengers don't fly a light twin - expecially in IMC or at night - without some training first, whether or not it's mandated by the CAA.