I do it all the time when traveling. If I want an AL40, I have to bring one with me, but AL80s are found at every destination. I bring a sling-kit and spare regulator only (technical diver style). For me, it isn't so much a calculation of how much gas I need as it is the availability of the AL80, and I actually prefer the way the AL80s ride underwater. The extra gas leaves you a lot more options on dealing with someone else who is having a bad day. For that occasional deeper dive where the extra gas is nice (but within recreational limits), sometimes my wife will jump in with it, use a third of the gas, then pass it to me. I'll use my third and then stow the cylinder. We use it at the beginning of the dive and leave it with 1000 PSI left in it; that way we both have plenty of gas on our backs for another diver, and I also have the option of handing off the AL80 to a diver needing gas.
The other way we'll do it is to each take a slung AL80, breath them down to empty first, and then switch to our back gas if we need more. For the second dive, we just swap out the slung AL80 for a fresh cylinder, but re-use our back gas (the sling is a lot more convenient to change). Sometimes we'll even have different gas mixes on our back and in the slung AL80, but you should be trained as a technical diver before attempting this.
Remember that you can breath the slung AL80 down to 200 psi, but you need to keep enough gas on your back to help someone else. Don't forget the implications of the slung cylinder only having a single second stage regulator.
At home, I have plenty of AL80s, AL40s, and AL30s. I typically grab the AL30s for recreational diving. Easier on my back.
