This works
@startuml
rectangle one <<one>> as "ONE"
rectangle two <<two>> as "TWO"
one -[thickness=8]>> two
@enduml
However as soon as I add style tags, the default style of the diagram changes and I lose the thickness of the arrow. But I don't know how to specify arrow thickness using the style tag
@startuml
<style>
{
}
</style>
rectangle one <<one>> as "ONE"
rectangle two <<two>> as "TWO"
one -[thickness=8]>> two
@enduml