Hi
I don't know much about keeping two Syrians together because I've never done it so I wouldn't know about if they hide their suffering. However I found this article:
Hamster Body Language | Hamsters as Pets
It says that when they feel threatened they will urinate, flatten close to the ground, attempt to run away, bite and many other things. You could try asking the people to look out for these signs even though their hamsters are "doing great together"(sweet talk them so they don't get so up tight about it).
I know it can be frustrating but some people just have to learn the hard way.