Your stepfather can use both. For any specific medical treatment, he can choose to use either the Medicare or VAsystem. And sometimes he can use both for the same treatment. Here's how it works. If he's treated at a VA facility, Medicare cannot pay any of the copayments that the VA charges him. Similarly, if he's treated by a non-VA doctor or medical facility and the bills are submitted to Medicare, he's not allowed to submit to the VA any portion of the bills that Medicare fails to pay.
There's one circumstance in which he could get covered by both systems for the same treatment. The VA might authorize your stepfather to be treated by a private doctor or facility, with the VA paying for the treatment. But the VA payment might not cover the full amount of the bill. If there's a portion of the bill that the VA doesn't pay, it can be submitted to Medicare and Medicare might pay a portion of it.