There is more than one address in a wallet. We can receive different payments using these addresses. But all the pays are in the same wallet. When we make an address payment, we will send our through which address.
For example, MyWallet is my purse. I have my A, B, C, D addresses in this wallet.
A: 0.2 BTC
B: 0.1 BTC
C: 0.7 BTC
D: 0 BTC
MyWallet: 1 BTC
I want to send the address bitcoin to D. Now when I write address D in my wallet, 0.5 BTC will be sent from which address.
I also have the same situation but compared to you i only have two address. I am using blockchain.info wallets and I am the fixed address and the changing address. But there are actually two wallets that there is on my account. In my experience there are options as to where will you charge the bitcoins that you will send. You can select from where or which wallet are you going to will take from.