The reason it came up as 1 ticket for 2 people, if probably that you gave her age. Children under 15 travel free if they are with a parent and shown on a full fare adult ticket. However, the least expensive way to do that is with a Bayern-Ticket, which costs €28 and is valid for 2-5 people for unlimited travel all day long (from midnight weekdays or 9 AM workdays, until 3 AM the following mornings).
A Bayern-Ticket is valid in 2nd class on regional trains. 2nd class in Germany is completely adequate and regional trains make the trip to Salzburg in only ½hr longer than the fastest express trains.
You can purchase the Bayern-Ticket the day of travel (they are unlimited in quantity) from any German Rail or MVV (Munich metro) ticket automat. If you buy one at a ticket counter, they will charge you an additional €2 for personal service.