摘自Jenkins文档：

expression
Execute the stage when the specified Groovy expression evaluates to true, for example: when { expression { return params.DEBUG_BUILD } } 
Note that when returning strings from your expressions they must be converted to booleans or return null to evaluate to false. 
Simply returning "0" or "false" will still evaluate to "true".

因此，如果您插入一个有效的groovy语句，它应该可以工作。
或者，您可以使用allOf语句，例如：

when {
  allOf {
    expression { params.x == true }
    expression { params.y == true }
  }
}