With one server you just insert data locally. After you added the second server you started moving data over the network. Apparently putting data into local RAM is much faster than delivering same date to a remote node.
Ignite is first of all a distributed system. I suggest this: configure your data loader node as Ignite client. Measure performance of loading data into one server node (1 server + 1 client cluster) and then measure performance of loading same data into two server nodes (2 servers + 1 client cluster). You will get linear performance increase.
