Baikonour
I have some experience with operating light twins and singles, my experience does not match your comments. Sure a twin is going to cost more to maintain but not eye opening figures like you suggest. Perhaps aircraft cost structures are different in the UK.
One thing that does need to be recognised is a light twin will have more systems/avionics than your average single so there is a cost associated with these. However if you compare with a high performance single (IFR, retract, etc) then you are comparing apples with apples and the overall cost difference isn't significantly more.
When it comes to some of the high performance singles that have been mentioned, I'd rather run two small Lycomings than one large Continental. Have a look around to see if the Continentals generally go full life or see what issues they have mid life and then compare to the small Lycomings.
maehhh
I believe Steep Turn is being unnecessarily alarmist.
His experience doesn't match what I see. I see no reason why you would not be able to buy parts for a 1980's PA44, they're still making this model.
There's plenty of PA28's running around without any problem getting parts. One thing Piper was/is very good at, and that is where practical, using the same parts in different aircraft. The PA44 is basically a PA28R airframe with two common O-360 engines.
By all means do you own research, but I don't see the PA44 becoming a aircraft where parts are a deal breaker any time soon.