ORA-15414: Appliance mode requires at least 3 failure groups. Is it possible to create ASM disk group on two storage cells or we need 3 cells?

Let say you have starting configuration:

The documentation don't clearly say: is it allowed to buy 2 storage cells and create separate diskgroup on it or NOT ?  And if you have bought two Exadata storage cells is it possible (and allowed and supported) to create new ASM diskgroup on these 2 cells (with Normal redundancy)? 

Or you need to buy at minimum 3 cells? Some peoples claim: you need at minimum 3 cells to create new disk ASM group in Exadata. But look the article https://www.oracle.com/technetwork/database/exadata/maa-exadata-asm-cloud-3656632.pdf, it allow 2 cells configuration:

 

Naturally this question is ASM-specific, not the Exadata-specific. You're allowed to take 2 cells and create separate ASM diskgroup with NORMAL redundancy on them.

Let see the 2-cells configurations. For example: customer has cells with 8T disks and have bought 2 cells with 14T disks. Then it can have 2 configurations:

 

or

The customer should choose which configuration suits it the best.

 

But if you create Normal redundancy ASM diskgroup you'll get an error:

 SQL> CREATE DISKGROUP data NORMAL REDUNDANCY
DISK 'o/*/DATA*'
ATTRIBUTE 'content.type' = 'DATA',
'au_size' = '4M',
'cell.smart_scan_capable'='TRUE',
'compatible.rdbms'='11.2.0.3',
'compatible.asm'='11.2.0.4',
'appliance.mode'='TRUE';

*

ERROR at line 1:
ORA-15018: diskgroup cannot be created
ORA-15414: Appliance mode requires at least 3 failure groups and either 10 or 12 disks in all failure groups.

The reason in appliance.mode=TRUE.
So the solution here is  'appliance.mode'='FALSE'.

You should to understand: 2 cells and disk group on 2 cells is allowed and supported but not recommended: