In SOAP, xsd supports Choice element and one of the element is mandatory.
It seems that the same can be achieved in OAS 3.0 as per documentation
The problem to use OAS 3.0 is lack of tooling support including SOAPUI.
However, how can the above be achieved in Swagger definition in 2.0.0?
'anyOf', 'oneOf' and 'not' were added in OAS3 and did not exist in previous versions. If you need to need to use them, then the only option is to stay with OAS3.
Unfortunately, I don't know. I lead the Swagger projects, but not quite involved in SoapUI. Perhaps it'd be better to ask for more information in their forums.