As has already been hinted at above, Amtrak fares have two components, rail fare and an accomodation charge. Both components go up in “buckets” like the airlines, with the cheapest buckets sold first.
For coach, there is no accomodation fare, just the rail fare.
For business class on corridor trains and first class on Acela, there’s an accomodation charge in addition to the rail fare.
For sleepers, there is an accomodation charge for the sleeping compartment and a rail fare for each passenger. Amtrak does not sell single beds, only small two-bed rooms, large two-bed rooms, and on some trains family rooms. A lone sleeper passenger pays the same accomodation charge for his room as two passengers together, though each is paying his own rail fare.
The key point regarding the OP’s “trick” is that Amtrak automatically charges the lowest-bucket rail fare for sleeper passengers, even if that bucket is sold out in coach. So if you really did meet someone on the train, or your companion already has paid rail fare and happened to get the lowest bucket, then no harm no foul. But if your companion hasn’t bought their rail fare already, you’re actually better off to put them on the reservation from the get-go.
In sum, the “trick” doesn’t work: at best its a wash if the companion got lowest bucket as a lone coach passenger, but its more likely that your companion will have paid a higher-bucket fare as a coach passenger than if reserved as a sleeper passenger.