Hans -

I have never heard of or seen any 'great formulas for T/Ws for multi engine models'. I have personally been looking at model airplane T/W ratios for a long time. and I can tell you of my findings.
For determining a value for thrust T, I put the model on a large flat table and attach a scale to the tail of the airplane and take scale readings when the engine is at full power. (No correction for wheel or table friction).
For W, I just weigh the model. ( In hindsight, I did not fill the gas tank on gas models).
My observations of performance with respect to T/W:
At T/W = .25 some fly and seem OK, but most seem underpowered.
At T/W = .4 allow takeoff from most surfaces and flies reasonably well.
At T/W = .6 will fly with authority and some aerobatics
At T/W = .8 good all-around performance.
At T/W = above 1 will go straight up, do 3D flying.

Don't think it makes any difference whether it's a twin or whatever.

feihu