File sharing Clustering

hi

i have a san and 2 file server

i want to know what is microsoft solution for these servers to work Active/Active in cluster with a shared storage

i read some document about the DFS and network load balancing but i do not understand how can done this

for example if  user1 connect to server 1 and open file.txt from there

and at the same time user2 connect to server 2 and open same file(file.txt) from that server

and both of the users modify the file and save this

what will be happen?does some section of the modified file lost?

i do not want to lost any thing

please help me to select the best solution