RE: Twinstar?????
2Slow, There is absolutely nothing wrong with being attracted to multi's. They can be a great sense of satisfaction. Understand there is this great aura about multi's and an attitude that it takes a super pilot. Nothing could be further from the truth and in fact they are quite simple to fly in normal conditions 'but' in an engine out situation they require an immediate corrective response if the airplane is to continue flight on one. It is this little understood response that gets so many into trouble with multi's. That response simply is to: Identify the dead engine and to apply rudder (lots) into the good engine side. With more powerfully engined multi's also some power reduction of the good one will help. This of course is assumes that you don,t wish to cut all power and make like a glider.
Most any reasonable twin is quite capable of maintaining flight on one if it is properly set up and the pilot has versed himself in what I described above. Many RC multi's are also capable of single engine takeoffs. a challenge sure but none the less can and has been done by many.
It sounds as your plan of action and progression in skills is sound and I applaude you for that. It also excites me that you want to acutually build something, a rather depressingly rare occurance these days. You would be shocked at the 'shops' folks have used out of desparation, My best story is building three meter gliders on a G.I. footlocker in the barracks many years ago. Persevere you can and will find a way.
Your choice of the Sig Seniorita is also fortunate. It simply is a superb small trainer aircraft that flys better than all the modern types and is very, very light. Its a simple build but takes takes some effort. Now after you get a little more time under your belt this same airplane will 'bash' into a fantastic little twin that will fly very well indeed as a twin. Very little cost also, you could use inexpensive little fifteens such as the TT 15, AP 15 and others. The bash is a modestly easy buy using simple underwing nacelle tubs, fair the nose in and some additional wing sheeting.
The photos below show the tubs that I have used now on two twins, two quads and working on a twin cub (single fuselage). The tubs are indentical on all these airplanes. Also is a photo of one simple trainer twin bash that uses a pair of .20's that flies wonderfully it was a forty size airframe.
Enjoy, John