And with respect to your second question "In what scenario I can go for splitting QlikView services on different server?":, you should consider this simple strategy: QVS (the QlikView service that manages documents, licenses and creates the document view in the AccessPoint) tries to grab as many CPU cores and RAM as possible to provide the best analysis experience to the end-users. The QDS (the reload scheduler that launches document reloads at predetermined moments that are specified in the QMC tasks) tries to do exactly the same: grab as many CPU cores and RAM to perform the reloads in the fastest way possible (i.e. without any slowdowns). As soon as they start competing for resources at the same time, slowing down reloads to hours instead of minutes and/or slowing down AccessPoint reaction times to a crawl, your server is in trouble.
You can solve this situation in two ways:
If possible, add more resources to the same machine (if either RAM or CPU are under heavy load, and you have empty slots left in your server machine), or
relocate the Publisher to another machine that is I/O-optimized (=speedy disks)