If I expose IQueryable from my service layer, wouldn't the database calls be less if I need to grab information from multiple services?
For example, I'd like to display 2 separate lists on a page, Posts and Users. I have 2 separate services that provides a list of these. If both provides IQueryable, will they be joint in 1 database call? Each repository creates a context for itself.
It's best to think of an IQueryable<T> as a single query waiting to be run. So if you return 2 IQueryable<T> instances and run them in the controller, it wouldn't be any different than running them separably in their own service methods. Each time you execute the IQuerable<T> to get results, it will run the query by itself independent of other IQuerable<T> objects.
The only time (as far as I know) it will make an impact if there is enough time between the two service calls that the database connection might close, but you would need a considerable amount of time in between the service calls for that to be the case.
Returning IQuerable<T> to the controller still has some usefulness, such as easier handling of paging and sorting (so sorting is done on the controller and is not done on the service layer which doesn't necessarily care about how data is sorted or paged). This isn't a performance concern though, and people will disagree about if it's best to do this in the controller or not (I've seen reputable developers do this and give well thought out reasons why).