Appointment of arbitrator

facts of the case, In this case, the defendant has unofficially appointed the arbitrator and later on going to confirm his appointment but they have notified to plaintiff about the appointment of an arbitrator. In meantime, the plaintiff has appointed an experienced retired justice of HC. The defendant now thinks if he could change the arbitrator instead of 1st one. issue- 1) Can the defendant appoint another arbitrator? and will it be valid? if yes any case law on similar ground. 2) The objection raised by plaintiff regarding another appointment, will it be valid? thankyou.