When you use additional polling engines, all the data collected by all the polling engines is written to the same MS SQL database. This would apply to NPM, NTA, SRM and SAM. NTA also uses an additional database called FSDB (Flow Storage DB), which is also used by all the polling engines.
DPA would be a separate install. If the latency between DPA server and the databases isn't bad, you can monitor it from a central DPA server. If not, you can install multiple DPA servers, and activate parts of the DPA license on each DPA Server. DPA comes inbuilt with a feature called DPA Central that allows you to centrally view the data collected by multiple DPA servers.
Thanks for the reply. since the primary and Additional polling engine will use the same database, will it impact the performance of the database? let say i have about 2000 devices that need to be monitor, will it impact the performance? since it is recommended that the polling engine only manage 10k component, will it apply to the database as well? will it consume much bandwidth for additional polling engine to communicate with the database?