1 parent nodetype with 2 subnodetypes - show all subnodes in a viewlist
You will have to use Relationships to pull in data from your other nodes. When you have defined your relations then you will be able to use these in each fields setting:

Relationships:
field_1a 
field_1b

Fields:
// Uses no relation
// Will display title from content-type 1
Content: Title
// Relation field_1a
// Will display title from content-type 1a
Content: Title 
// Relation field_1b
// Will display title from content-type 1b
Content: Title