We have a forest "Domain1.com with SCCM 2012 R2 installed. This forest has a Trusted relation with another forest "Domain2". "Domain2" is "Single Label Domain"

1) Could I discover computers on "Domain2" Domain??

2) Must I configure "Domain2" Domain as "Disjoint Namespace" ??

3) Must I configure something on "Domain1.com" ?

Why don't you just try it? Discovery is a read-only process when it comes to AD.
1: yes
2: not sure
3: no

I have some doubts...

http://technet.microsoft.com/es-es/library/gg682077.aspx#BKMK_SupConfigSLD

Except for out of bandmanagement, Configuration Manager supports site systems and clients in a single label domain when the following criteria are met:

• The single label domain in Active Directory Domain Services must be configured with a disjoint DNS namespace that has a valid top level domain.
  
  For example: The single label domain of Contoso is configured to have a disjoint namespace in DNS of contoso.com. Therefore, when you specify the DNS suffix in Configuration Manager for a computer in the Contoso domain, you specify Contoso.com and not Contoso.

1) it's NOT possible. Discoveries don't work (with computer account or user accounts)

2) I'm sure that I have to configure DNS on "Domain2" because is a Single Label Domain. This action requires applications testing.

http://technet.microsoft.com/es-es/library/gg682077.aspx#BKMK_SupConfigSLD

3) I'm not sure.

Hi,

Please make sure the specified account has Read permission to Domain2.

 And here is a blog about discover computers in another trusted domain, although it is for SCCM 2007. Hope this could be helpful.

SCCM | Discover Another Trusted Domain

Best Regards,

Joyce Li