If I have two different travel passes loaded, can I travel on a bus that my second travel pass is valid for, even though it is not active?
No. You can only travel with a travel pass on services that your active pass is valid for (you can only have one active pass at once).
If you travel on a bus that is not valid for your active travel pass when you tag on the Snapper reader will deduct the minimum boarding fare from the money loaded on your Snapper. When you tag off the balance of the fare for your journey will be deducted from the money on your Snapper.
Even if you regularly travel using a travel pass on your Snapper, its a good idea to also have a few dollars loaded onto your Snapper for emergencies.