In another Topic dealing with small bucklers the way the buckler was worn on the person over the handle of the sword led me to wonder how well this would work with a sword with a long handle and this led me to wonder if the longsword or bastard sword was commonly used in combination with a buckler or is sword and buckler only practical with a one hander ?

I have seen pics of historical illustrations or of renactors were a buckler was worn and the sword seemed to be a longsword and not the more usual 1:33 buckler and one hander combination ?

Oh, and finally, can a reasonably light longsword be used effectively with the buckler from a technical swordsmanship perspective ?

Yes,

Please see Paulus Kal's fechtbuch (among others).

Duerer's artwork also shows a number of instances where long-handled swords are being wielded with shields.

Yes, it is certainly doable. It is depicted in historical artwork and is still in practice by certain WMA groups today.

The only other thing I can add is that I don't think that long sword and buckler would have been commonly used, but as others have stated, it certainly was used.