If you're going to double up tanks, then yes you really should use an isolating manifold.
Alternative is using side mount tanks and switching regulators.
Another alternative is to get a larger single tank, or carry a sling (bail out) bottle as an emergency reserve... sometimes called a pony bottle. Sometimes its strapped to the main tank, other times it's just sling under your arm.
If your profile is correct, you're only have a couple dozen dives under your belt. As you progress and get better, your air consumption will come down. I know mine did.
I went through all the issues with air consumption and figured I needed bigger tanks, double tanks, etc. Nope. What I needed to do is dive-dive-dive... get out there and practice. My air consumption went down about 20% after dive 50... and another after dive 100... and a bit more after dive 150 and 200...
If you're serious about getting more air, I'd go with a larger single tank. Not a pony bottle or sling bottle or doubles... simply a larger single tank like an HP120 or LP108 or something... it'll do 2 things... 1) give you more air available to you now and in the future; and 2) take some weight off your belt or out of your pockets as the larger tanks are heavier.
Good luck!