Migrate multiple Magento 1 stores

Hey there! 
I would like to know more about the process to migrate magento 1 website. Let me explain the problem. 
We actually got two magento 1 website : 

• Magento A : version 1.9 Single store
• Magento B : version 1.7 Multi-stores (2 stores)

What I need :
Having only one Magento 2 multi-store with the both A+B magento (3 migrated stores in the end on one magento 2) 
Could you explain me the process ? Did I need to check keep original IDs anywhere ? Or not ? I don’t get it. Do I need to create stores manually or anything ? I really need help on it…
What I got :

• 1 sql drump hosted for the magento A (the magento 1 multi-store)
• 1 sql dump hosted for the magento B (the magento 1 single store)
• 1 empty Magento 2 application ready to use
• 1 wonderfull UB Migration tool pro extension <3

Could you explain me more about the process and how to process to get my final Magento 2 with those 3 stores on it ?
 
Thanks a lot for your help!
Laurent