(how) can multiple indexes be amalgamated?

I have fscrawler running on a single virtual machine running ubuntu 19:04 on a Synology NAS

At the current indexing rate, it will take 10 weeks for the index to complete

If I run fscrawler on multiple virtual machines, with each VM indexing a fraction of the total files:
1 - (how) will it be possible to amalgamate the indexes?
2 - Is there likely to be a time saving, though all the files being indexed arestored on the same volume?
3 - can you suggest how much RAM to allocate to each VM as this will determine how many VMs I create

Is 'Rollup jobs' the answer to question 1? (https://www.elastic.co/guide/en/kibana/current/data-rollups.html)