Is it possible to migrate and merge multiple M1

Hi Laurentius, 
We’d like to answer your questions below: 

Is it possible to merge multiple M1 webshops into single M2 multi-store with UB data migration pro? Have this ever been tested before?  What are the implications? would there be any foreseeable difficulties? 

Yes, our migration tool allows to migrate multiple M1 webshops into a single M2 instance. However, there’re a few things you should notice before proceeding (suppose that you migrate 2 Magento 1 databases only): 
1. When you start migrating from the 2nd Magento 1 database, it’s up to you to enable the Merge Default Website option or not (this feature is explained right in your migration dashboard like this one)
2. Beside, due to our migration working rule, if you want to keep original IDs of specific data objects, your M2 instance must be a fresh instance. That means you can’t not use the Keep Original IDs setting when migrating from your 2nd Magento 1 database (you can learn more about the Keep Original IDs setting here). 
3. The delta migration feature is available for your 2nd Magento 1 database only. 
4. Our current license grants you permisison to migrate from one single Magento 1 database instance only. Each extra M1 db will need additional license.

Would UB help in merging the stores together?

As explained above, if you enable Merge Default Website option, it will help to merge your default Magento 1 website (including default store of default website and default storeview of default Store) into existing default Magento 2 website.
 
If you mean other custom mapping, the customization will be needed. Please advice how you wish to map your M1 webshops to M2, we will get back to you then. 

I really would like to know things such as how to handle sales data / customer data (possible duplication).

Duplication issue will not occur. 
Regards,
Ubertheme team

Hi Laurentius, 
 
Just want to add extra note that if you migrate multiple M1 databases into a single M2 instance, the migration workflow will be a bit different as follows: 
 
Step 1: Install a fresh Magento 2 instance and our UB Data Migration Pro extension 
Step 2: Migrate from your 1st Magento database
Step 3: Verify all migrated data ready for use
Step 4: Clean all migration log tables. We will provide you instruction to clean these logs. 
Step 5: Migrate from your 2nd Magento 1 database as normal. 
 
Further, please make sure you obtain license for each new Magento 1 database you plan to migrate. 
 
Regards,
Ubertheme team