There is potentially a causal chain leading from high initial infectious dose to high viral load to high disease severity for COVID-19. See, for example, evidence surveyed here.

However, once someone has been infected with a given infectious dose and is displaying a certain viral load and disease severity, is it possible to receive further infectious doses that can increase viral loads and disease severity?

I ask for the sake of informing public health guidance: Say an elderly couple have both been exposed to COVID-19 and have developed mild symptoms. Is there any reason that they should isolate from each other to avoid the risk that they each keep exposing the other person to further infectious doses and potentially worse outcomes?

Do we have any evidence on this from other coronaviruses/influenzas?