On using a small tank to increase bottom time and provide redundancy

[OMyMyOHellYes]

I figure it won't make any difference. Although from what I've read dropping below 100 psi at depth might result in water entering the tank. So I try to stay above that but I'm not always successful.

40 M /132 ' = 5 atm of pressure (or for the metrically inclined, that's be 5 bar on the spg) for us others that the pressure at 130' would be 73-74 psi trying to get into the tank so if it's more than that, then well water ain't getting in. Now what your reg will will require to maintain its 140 PSI IP, well ...