One of my biggest concerns is the idea of using dissimilar tanks (the woman going smaller) to try to solve a non-existent problem in recreational diving.
Rock bottom is the minimum amount of gas to get two divers from maximum depth to the surface. In the United States, we use cubic feet to measure tank volume. Let's assume a dive is planned to a depth of 100 feet. To determine rock bottom, we assume it will take 1 minute for a team to recognize and deal with an out-of-gas emergency, calm down, clean up any dangling gear, and begin to ascend. Then, we need to plan for a 30 ft/min ascent from 100 feet. 100 / 30 = 3.3. We'll round that up to 4 minutes to ascend for safety. Do we want to plan a gas-sharing safety stop? Or, do we just want to reach the surface and assume DCS won't be a concern?
If we plan for a safety stop of 3 minutes that will be 1 min to handle the OOG emergency + 4 minutes to ascend + 3 minutes safety stop = 8 minutes of gas. We have two divers sharing gas, so that will be 16 minutes of gas. We need to adjust consumption for depth, so we'll take the average depth of 50 feet and convert to ATA [(Depth/33) +1]. (50/33) + 1 = 2.5 ATA. 16 minutes of gas at an average depth of 2.5 ATA = 40 cubic feet of gas. That also assumes a 1 cuft/min stressful breathing rate in an emergency. YMMV.
Both buddies have to reserve 40 cubic feet of gas for the other. It doesn't matter who is using what cylinder as long as each cylinder has 40 cubic feet for a gas-sharing ascent. If a wife had an AL63, the turn pressure to reserve 40 cuft would be 1,900 psi. If the husband had an AL80, the turn pressure would be 1,500 psi. If the wife happened to hit 1,900 before the husband hit 1,500, she'd thumb the dive. "Up!" If hubby hit 1,500 before she hit 1,900, he'd signal "Up!" So, if they both used the same tanks, say AL80s, who cares if the wife has 2200 in her tanks, when her husband hits 1,500?
Danger comes when the wife's tanks are small capacity and the team hasn't planned properly for the minimum gas to ascend. If the husband runs out of gas, there might not be enough in an AL63 or AL50 to get them to the surface. An old rule of thumb was to take the depth you are at and add a "0" and that will be how much gas you need to surface. At 130 feet, you'd need 1,300 psi. Then, multiply by 2 to calculate someone breathing off your octo. 1,300 x 2 = 2,600 psi turn pressure.
If we figured out rock bottom at 130 feet, we would need 1 min/emergency + 5 min ascent + 3 min safety = 9 minutes of gas x 2 divers = 18 minutes of gas at an average of 65 feet (2.9 ATA) = 53 cubic feet of gas or 2,600 psi in an AL63. If the husband is bad on gas, wouldn't it be better if the wife had an AL80 at that point?
You can use that zero trick as you ascend. If you were diving a wall at 130 feet, when you reach 2,600 psi, it is time to ascend. But if you ascended to 100 feet, you could dive until you reached 2000 psi. If you go to 40 feet, you can dive until you reached 800 psi. Regardless of depth, you don't calculate below 500 psi. Now, keep this in mind! This is a very loose rule of thumb and you need to verify that it will work on the fly for your consumption rate and cylinder size.
