I purchased an e-ticket for two people from DB Bahn via a travel agency. We are travelling through two Dutch train stations. I know I have to scan it in at these stations. Since the ticket is for two people - do I scan it in twice? Or do we both go through on the same ticket?
Print two copies of your e-ticket, so you can each go through independently. This may have the added advantage that if one of you visits the loo or otherwise needs to walk through the train, you can take a copy of the ticket with you. You might want to do that anyway, as the train manager may ask you to present your ticket at any time (at your seat or when walking around), although technically only one of you can carry a valid ticket as the ticket is only valid with matching id.
Scanning printed tickets on Dutch railway stations is purely to open the gates. So if there is no gate you do not need to scan, it even says such on the Dutch (NS) self print railway tickets.
Scanning the ticket will open the gate, usually long enough to walk through with two people. You can then scan the ticket on the inside of the gate (be sure you have a gate with a green arrow, not with a red cross on it). But if you prefer you can hand the ticket back over the gate to scan again for the next person. (Or as @gerrit suggest in the other answer, print two copies.)
Tickets with chips and chip cards need to be scanned every time you change rail companies as well as every time you leave and enter a station. It is, however, the chip that remembers the action and tickets without a chip do not register being scanned nor does the system remember seeing your ticket.
Remember to use the scanner (at the gates) which have a little window, not the ones with just a touch pad.