The 1t/2t is how many clocks the memory controller spends sending a given command to the ram. 1t means that the controller sends the command on one clock cycle, 2t means it sends it for two clock cycles.
The end result is that running 2t instead of 1t is like adding 1 to nearly every ram timing.
For instance, running 1600-8-8-8-1t and running 1600-7-7-7-2t are very nearly identical, while 1600-7-7-7-1t is significantly (in benchmarks, at least) faster than either one.