Can I stake two instances of the same wallet at the same time?

Yes, you can stake with two instances of the same wallet at the same time. However, there are caveats. If you use the wallets for more than just staking and choose to generate a new address by generating a new key pair in one of the wallets, then the other wallet will not have a copy of that key pair. Besides this, you are exposing two machines to potential attackers as a way of getting to your hot wallet instead of just one. As is always the case in crypto, you are responsible for the safety of your own money, so it is vitally important you are careful with your security. So long as you bear these caveats in mind, it is possible to stake in this way.