Unfortunately this isn't possible in MindManager - this is one area where XMind has the advantage. There is however a workaround, which I posted a few months ago in response to a similar query: https://community.mindjet.com/mindjet/topics/create-boundary-around-area-of-map
Essentially you create an "invisible" parent topic for the two topics. This is a dummy only with no text or fill colour and a line only for topic shape, which is in the same colour as the sub-subtopic lines. You then create a boundary around this parent topic, which will then cover the two visible sub-topics.