On our scenario, we want to map two properties from the source, not one.
However, we do not want to merge them into one because it's not the case that two existing properties will map to one property; they will still be a one-to-one mapping.

We would like to do something like this,

$propMappings = Set-PropertyMapping -MappingSettings $propMappings -Source "prop1" -Destination "prop3"

$propMappings = Set-PropertyMapping -MappingSettings $propMappings -Source "prop2" -Destination "prop3"

However, by doing this ShareGate overwrites the first mapping with the second one.

The reason for doing this is simple: each content type will only have one of the fields.
If we use single mapping, one of them is ignored.
If we use multiple mapping, ShareGate will try to merge the two properties that exist at the list level, even though they do not exist on both content types.
This creates a series of problems from default values to forcing the destination property to allow multiple values when none of the 3 properties (2 source, 1 destination) are multi value.

A potential solution for this would be something like,

$propMappings = Set-PropertyMapping -MappingSettings $propMappings -Source "prop1" -Destination "prop3" -ContentType "admin"

$propMappings = Set-PropertyMapping -MappingSettings $propMappings -Source "prop2" -Destination "prop3" -ContentType "meet"

In this alternative, we could specify property mappings that are content-type specific, which would allow a one-to-one mapping in a scenario where multiple content types exist and we want to control what field goes into the destination field depending on the document's source content type