Making an isolation transformer from two transformers
Yes, it is doable.  There's a slight problem that if you were to take two identical step-down transformers and connect them the way you're planning, the output voltage would be less than the input voltage.  For good quality off the shelf transformers I would expect maybe a 5 or 10% drop -- for salvaged transformers, I couldn't say.
You can judge the power handling capability of a transformer by what the light that you took it out of was rated at.  If the light was 10W, the transformer is 10W.  Another way to get an educated guess is to weigh them, and compare with transformers of known weight and power capability.
I'd connect things up so that the smaller transformer is on the output side, because some energy will be lost on the input side -- you want to dump that heat into the bigger transformer.
I'd also make sure as I was using them that they're not getting too hot.  If the overloading is mild, the transformer will just quietly overheat.  After making sure you won't get electrocuted, feel the transformer as you're using it, starting from cold.  If you can put your hand on it and hold it there, it's fine.  If it gets too warm to touch, it's too hot.  (Note that you don't want to just let it work for an hour and then check without being very careful -- if you haven't touched it for a while, approach it like your finger will go &quot;tssss!&quot; when you touch it).