How do I connect two identical batteries to increase the consumer lifetime?
When you connect two batteries in parallel, you are effectively halving your discharge rate while doubling your capacity -- effectively, doubling battery life, as you are intending to do.

Serial connection does not increase life, but rather, increases operating voltage, which some devices need. If your device has two serial batteries, then you need to connect another two serial batteries in parallel.

If they are not going to be recharged in the configuration you are putting them in, then you should be able to connect them in parallel. They will end up using a small amount of power by charging each other a small amount, but you will probably see more than a doubling of battery life.

If you are pulling a lot of power from a battery, you will find that its battery capacity decreases and causes excess current draw to decrease life and decreased capacity to decrease life. There are some excellent explanations about batteries online. The link I gave is more lithium focused, but the general knowledge applies to most batteries. This link is specifically about discharge rates and their effect on battery life.

Hope this helps. If you comment on anything that is not clear, I will edit and expand on it.