Possible to use default user picture in rewritten views field?
You can use no result behavior option to show the default picture for the user's image.

Here is my screenshot : 



If you want to combine two fields then you have to take two field firstly in view fields and shown their output in diffrent-2 field after that you can combine both fields in new field using rewrite option. The first two field output is hidden who's value you want to combine.