You wouldn't need to add much more sugar if you are adding twice as much extract. Adding more sugar will depend upon what you are looking to do. I would recommend seeing what the OG is going to be with an online tool (personally, I like Brewer's Friend). It would all depend upon what you are aiming for. Adding a lot of sugar could impart a very strong alcohol taste to the beer which you may not want.
This would all depend on the style that you are creating. Since you are adding more malt you could cut back on the dextrose (maybe 0.5 kg) due to the added sugars from the other malt kit.
You would be able to get by with only one yeast packet but using both would give you more yeast and get the fermentation going quicker. This is assuming that the yeast is the same strain. If they are different, it could make things interesting and that would depend on what you are aiming to make with the batch.
Check out this question for mixing yeast strains. Using Multiple Strains of Yeast in the Same Batch?