Can two teams work on one product backlog?

Scaling does come with a cost, so I can't advise whether that is the right decision; but it's a totally valid approach.

And if there is just one product, it is much better to do things this way, rather than arbitrarily split Product Backlog Items across two backlogs, or between two Product Owners.

The two teams will need to co-ordinate between each other. The two Development Teams may come to arrangements about what each team works on, and the Product Owner is probably keen that any such arrangement does not prevent the most valuable items being done first.

Does this sound right? 

Are the team members who will now be available self organizing into 2 teams?

Consult the PO ask the purpose of adding new team, what value he wants to gain and overall picture.

There must be some roadmap to guide the PO which triggers the addition?

Assuming that you have one product, you can have multiple teams working on the same Product Backlog. The structure of one Product Owner per product (therefore, also, per Product Backlog) is also correct. Although designed for cases with 3+ teams, both Nexus and LeSS follow a similar scaling approach.

The issue in this case, though, is the impact of adding another team. Adding more people may not have the desired impact, if the intent is to do more work in less time. First, these 6 people will need to be hired and trained - this will take time away from the 7 people currently working on the product. Once onboarded, there is going to be a continuous coordination overhead between the two teams to ensure they are able to work on the same product without "stomping on each other's work". Scaling is a non-trivial problem.