Bath and sink connected on same pipe - bath gurgling when sink emptied
1:Can multiple bathroom sinks, baths, showers ect share a common drain pipe ?
Yes they can and do all the time.

If so are there any rules? of course there are the International Plumbers Code being one of them - and depending on what other rules your locality has adopted.

Now on to your issue : 


Did this problem just start (on previously working system)?
Is any of it new construction ?


Potential Causes:


Your drain from sink to drainage pipe is partially blocked!
Your vent is is either blocked or partially blocked!


Solutions:


Run a plumbing snake down the pipe towards the sewage drain.