There is a reason that virtually no one dives with a single post manifold anymore - you gain gas and gain the ability to getinto serious deco, but then lose the redundancy that will save your butt when your reg fails,a neck o-ring, fails, or a burst disc blows.
"Cheater bars" and "pigtails" were formerly used to connect two k-valve tanks together to get a pair of temporary doubles, but in addition to the problems associated with a single post manifold, it adds to more much weaker connections and the potential for a serious leak or failure at depth. They made sense in an era when the regulator represented the bulk of your equipment cost compared to all your other gear (mask, fins, snorkel, weightbelt, tank, and maybe a horse collar BC) and adding another regulator was considered un-economical. That argument really does not wash anymore when a new and reliable reg can be had for the price of a pair of tanks.
Independent doubles are a very good route to take for a traveling diver/rental tank situation. They allow both extra gas capacity as well as excellent redundancy (as long as you manage them approproately. Gas management normally involves using 1/3 of the first tank, switching to the second tank and using 2/3rds of it before switching back to the first tank to use the 2nd 1/3rd of it (and leaving 1/3 in each tank for a reserve).
The option suggested above of slinging another AL 80 is also sound. It is less comfortable in the water, and still requires the same gas management strategy as independent doubles, but it saves you the cost of a set of "travel" bands - normally about $120.) One potential downside is that the AL 80 with what should be "backgas" is competing for the same real estate as a(any) deco bottle(s) you may want to use and tends to lead to more clutter than you really need on the front side.
One more thing to consider, if you are considering deco diving, you need to get yourself some deco training, usually this consists of advanced nitrox and deco procedures training. This is doublely true if you are planning do deco dive while solo, where traiing and skills issues are doublely important.
In short, deco diving with a pair of 80's connected by a cheater bar used by a diver with no deco training is not much more than a form of assisted suicide.
