Multiple tasks on GTX 1080 Ti

I'll answer the bit that wasn't answered :-).

Yes, it does, and by enough to make it quite worthwhile if you're interested in increasing your daily output.  The biggest increase will come from going from x1 to x2 - ie. changing the GPU utilization factor from 1 to 0.5 in your project preferences.  Be aware that the change kicks in when new work is downloaded and not just by clicking update if work fetch doesn't occur.

For the impatient, the easiest way to guarantee immediate work fetch is to make a small (but sufficient) increase to your work cache minimum setting locally in BOINC Manager.  You can easily reverse this after downloading new work occurs.  Once the change arrives, a second task will start and BOINC will attempt to double the number of tasks on board because it now sees two tasks being processed where the estimate is still the old x1 value.  As soon as tasks with longer running times finish, BOINC will immediately correct the estimates.

With a 1080 Ti, you will be able to go higher than x2 but it will be a case of diminishing returns.  The only way to know what is best for you is to experiment and observe the crunch times at each setting for a sufficiently large number of tasks to get a good average.  Something like 20 tasks and see if most of the results are pretty close to the average value.  Also note that you will produce a bit more heat and consume a bit more power.  You will get a bit better efficiency, particularly if you try to ensure that all task instances aren't starting and finishing simultaneously.  It's fairly easy to stagger the start times for each task and because run times are fairly uniform, this tends to be maintained, once established.

The one thing to keep in mind is that each GPU task instance needs to 'reserve' a CPU core for support duties.  As you have a 16 core machine, this shouldn't be a problem.  If 15 of your cores are currently crunching CPU tasks, one of those tasks will stop temporarily when a second GPU task starts.  It might be worthwhile (through your compute preferences in BOINC Manager) to further limit the number of CPU cores that crunch CPU tasks, if you really want to extract the best performance from your GPU.

Whilst the GPU utilization factor is the easy way to do what you want if you don't need anything special, you can get more fine-grained control if you are prepared to master the application configuration system which uses a special file called app_config.xml.  Check out the link for the documentation.