Indeed it's much easier if you're proficient in both.

I've written a number of non-trivial IOS apps that are heavily server based. I've done both the server side (Ruby Sinatra API App, MongoDB, Varnish) and the client side (Obj-C naturally) and consider myself equally proficient in both.

The client generally takes much longer to write and debug than the server due to all the little issues with having a smooth, non blocking UI, network availability, juggling data and managing resources that you have with the client app. Not to mention any custom UI controls you make.