Baikonour: Again, I'd research this and try to quantify what 'only a little' means. If you think that only a little is due to just having 8 cylinders instead of 6, you may need to rethink. Having had a quick look at a random PA-44 POH on the web, it seems you're likely to see around 20USgph per engine - in an SR22 the POH shows around 16 (per engine - but now there's only one of them...) as typical. Of course both POHs are probably way off, but for comparison...
Did you bother checking your "facts" for reasonableness?
An O-360 burning 20 US gph or 75 litres per hour? Really? I'd suggest that figure is total burn for both engines. My experience of the O-360 is around 37 - 38 litres per hour
The figures I have for a 310 hp TSIO - 520 are 18.5 US gph or 70 litres per hour. I suspect the SR22 might burn a little more than your figures. I know I'm comparing two different engines, however they are from the same manufacturer and produce the same HP. YMMV
I'd say "only a little" isn't far off the mark.
Horsepower for horse power the fuel burn between a twin with say two 150 HP engines is pretty close to a single with a 300 HP engine.
Having said all that, fuel burn is a small part of the equation.
Baikonour: It will be at least twice as much as an Arrow. Also keep in mind that that can translate to twice the downtime when it is stood in the hangar and you are unable to use it...
Your logic escapes me here. Sure there's two engines and props, but there's only one of everything else, one airframe, one undercarrriage system.
There's no reason why it will be twice as much as an Arrow.
Same applies to the downtime. It's still only one airframe.