We ordered two phones, 6s+ RG (won't be shipped until at least Oct. 16th) and 6s 64gb silver ready to ship. The order status page says the 6s won't ship until the other unit is in stock, however you can call customer service to split the order.
The customer service rep said if he split's the order we would lose our place in line with the 6s+ RG. He offered two options:
1. $50 account credit
2. T-mobile will send us a loaner phone that is in stock (6s 64gb). When they ship the original order with both phones, we would need to return the loaner.
We decided to go for the $50 credit and wait.
I am sure you will need to call up to be offered either option.