cbind(SGDFone, SGDFtwo)

Sorry, I deleted one of the grids ... ;-)  

I will redo the analysis later and come back with the result of

 all.equal(getGridTopology(a), getGridTopology(b))

However, before I deleted the Grid I checked

  getGridTopology() for the two SGDF separately. They were the same!

I solved the problem with

 newGrid <- cbind(SGDFone[,,1:13], SGDFtwo[,,1:61])

this worked. Don't ask me why...  (1:13 and 1:61 are all columns of the 
dataframes)

Ingo