Players need to feel like they are playing in a world that allows enough interaction with other players while of course offering enough gameplay to be enjoyable on its own. This is where MMOs excel.
In Tower of Fantasy, can players play together while on different servers?
Unfortunately, you cannot play Tower of Fantasy with someone who is on a different server from you. Even if you are set to the same region as each other, characters are held exclusively to the server on which they were created. At the least as of this writing, there is no way for servers to crossover with each other.
Either one of you will need to start over on the same server, or both of you will need to go to a new server together and start over.
This is not an uncommon scenario with MMOs. Many of these large scale games have servers separated from each other because it reduces the stress of playing together.
The reality about having such large servers without a connection between them is that you could have tens of thousands of people on a server, but they are separated from one segment of the community. One way the blow here might be mitigated is if Tower of Fantasy ever has a mechanism to transfer characters. Unfortunately, this is not available at this time.