Hi Lewis42, the bahn.de website gives lot of information on bahn.bonus, but unfortunately only in german:
Link to bahn.bonus website
If you book a ticket for 2 pax and include your bahncard number, you will get the points (bonus and status) for the full amount of the ticket. In the past if you wanted to book a ticket for more than one pax the discount needed to be the same for all participants though (so in this case selecting bahncard 50 1st your partner would need to have the same type of bahncard) - not sure if this is still true. If you reach bahn.comfort status (basically via 2k spend within a year) you can take one person with you into the lounge. I don't see any other benefits out there for a partner traveling together with you.
I usually use my point for day cards for the whole network. Saturday daycards first class are 3.000 points, other days 4.000 points; day cards are valid untill 10:00 am next day for the whole network. Other good use: upgrade vouchers.