Merging two existing Jira instances is not a trivial task. It can be done, but it can be very complicated to do this. Jira Cloud does not have a means to import specific projects. That is why the KB you see here is documenting a means to do a complete restore of one instances into Server, and then use the server project restore method to import all those projects from B into A.
Only after you have all of these imported into a single instance can you then generate a new XML backup and import that into a single Cloud instance. It can be done, but you need to follow that KB you cited very closely. Ultimately you need to restore each Cloud backup to a Jira Server version. From there, you would then need to create a new XML backup of one of these instances in order to then use that XML backup as a source of a project import. This process is very tedious to complete even for a single project because you have to create all the projects, custom fields, and more before you can do the import. There is also a lot of other configurations that are not included via a project import so, merging two Jira instances can be very difficult to do seemlessly.
Many customers that have encountered this problem have sought out third party solutions to this complex data migration. As such there are a pair of different plugins for Jira that might be helpful in this case, I would recommend checking out:
Project Configurator for Jira
and/or
Configuration Management for Jira.
Either one of these could make the process of migrating this data much easier to complete.
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