Hi there! Only one phone per installment agreement is allowed with Boost Up. If you are referring to adding a secondary line to your account to join as a family plan, you can do so by inviting the secondary from your online account. You can access your account here. Keep in mind that on a family plan only the primary line can have a payment installment with Boost Up.
In order to assist you better. Can you please let us know, if you're trying to get 2 line under the $60 Boost® Unlimited Plus plan? If so, You can have 2 line for $100 on the $60 Boost® Unlimited Plus plan.
Understood. You can have a family plan of 2 lines for $100 on the $60 Boost® Unlimited Plus plan. For more details regarding this and other available plans, please visit Our Website. If there’s anything else that we can do for you, let us know.