Here is the fixed cap: http://dl.dropbox.com/u/6660860/fixed.cap
The first problem was caused by construct not knowing which building to change so it just changed both of them.
The second problem was caused by your TAB presses both getting detected in the same tick causing the variable to not change. (I think that's what the problem was anyway).
One problem i can't seem to fix though is that when you press tab for the first time after launching the preview, it doesn't work but it works fine for all the presses after :S.
And finally, i have no idea what version of construct you made this in but it was older than what i saved it in (v0.99.84). So you might need to download the latest unstable build to see my changes and then just copy them in your real cap .