It’s easy! Just change the way You think about the inner blocks as they’ll always have “only one place” to include the inner blocks. Is all about making more blocks + limit blocks that You can insert + default block template.
Make a container block with innerblocks and name it like “Two Columns: Layout A”
Make a child block called “Two Columns: Layout A – Left Column”
Make a child block called “Two Columns: Layout A – Right Column”
Now create template with lock function for each blocks mentioned above:
1. Container – template with allowed only: 2 and 3 (blocks left & right column)
2. Left Col. template with allowed only: ‘core/list’, ‘core/button’, ‘core/image’
3. Right Col. template with allowed only: ‘core/heading’, ‘core/paragraph’