If you take any of these 2 offers you will be entering into 2 handset installment plans (one for BlackBerry and one for Windows Mobile) therefore you will be paying 2 installments as well. Also, if you want to disconnect any one of the installment plans, you will be charged the termination fees as per the contract
Technically it is possible to have both plans activated on one sim card as IMEI mapping is not used.
On the benefits side you will enjoy the benefits of both the plans
PLEASE NOTE that you cannot keep the iPhone plan and the Windows plan in the same time