Onegaeshimas is not unique to the dojo. Japanese people use it all the time when making requests. Usually, one person - the requester - says it. But in the dojo, typically, both say it. However, when I did Judo juniors would run up to seniors and shout it out. The first one to reach the senior was the one who got to train with them - usually, but not always, as sometimes, the senior would ignore them and choose someone else nearby. In this particular Judo class - in Japan - the seniors never said anything. In an Aikido context it might appear rude, but as I already mentioned above, it is actually the norm in society outside the dojo that only one person says it - the requester. I also saw other Judo dojos that operated as Aikido ones in this respect.