It is a bit "Strange" - Commerce supports multiple payments but the most common scenario is when you use gift card + other payment methods, not two same payment methods. But it's just a matter of implementation because the framework does not really care the payment implementation detail, as long as it can call the payment gateway to capture/authorize the payments.

In your case the workflow would look like this:

• Add two payments with specified amount to the order form
• My memory is a bit rusty now but the calculators/workflows will only care about the "not authorized" payments. So if you are purchasing an order of $500, and with two payments $200 and $300, and the $300 is authorized but $200 is not then you can try again (either with same payment or replace it with new payment). If I am wrong here then you can modify the workflows quite easily.
• The default cart payment (Authorize.net) will just refund the payments if the order is cancelled (though I am not so sure). But, you can always implement your own payment gateway to handle the refund, as long as the gateway allows it.

Man, I know it is a strange one but the client likes it for a reason!

Thanks for sharing your thoughts Quan. I understand that it depends on our implmentation plan but were just looking for best practices if anyone had same requirement before.