Consider the following premises. t_2 <-> u. u -> s. t_2 <-> s. Can we infer s <-> t_2 from them?
 From t_2 <-> u we can infer t_2 -> u. From t_2 -> u, u -> s we can infer t_2 -> s. From t_2 <-> s we can infer s -> t_2. Finally, from s -> t_2, t_2 -> s we can infer s <-> t_2. Therefore, the answer is yes.