If you are dead-set on learning both, without a doubt learn SQL Server first. Access is not a bad tool, but it is grossly overused. If I typed a letter to my mother using Excel, most people would tell me I'm a moron, and I should be using a word processor instead of a spreadsheet. Yet I see people all the time who use Access when they should be using something more suited for the work. Just because you can (type a letter to your mom in Excel), doesn't mean you should.
Access developers tend to never evolve past Access. I've never understood why. Most of them are very bright, but strangely beholden to Access. It's a neat little tool, but it's a tool, like Excel, not a technology platform.
If you learn SQL Server, you will be learning an industry-strength database. It's good for the little stuff, and it's good for the big stuff. Moreover, you will be learning the foundations of all relational databases as well as a reasonably* compatible version of SQL, 90% of which should work on any databases. The remainder (date functions, editing tables, recursion) will be more DB-specific, but the 90% will get you off and running when you encounter a new database. I learned Oracle first and had no problems picking up SQL Server, Sybase, Postgres and MySQL.
If you learn SQL Server and decide you still want to use Access, then go ahead. Link Access to your SQL Server database and go to town.
Better yet, learn tools more fit for the job you want to do... Off the top of my head, I'd recommend .NET (C#). If all you're doing is queries and reports, just stick with Excel and MS Query. It's great and SIMPLE.
And, as others have noted, Access SQL is incompatible with every other known version of SQL. It renders to the actual SQL when you link to databases, but that rendering is not exposed unless you can intercept it on the database side. For example this:
select a.part_number, b.qty
from inventory.part_master a, warehouse.stock_locations b
where a.part_number = b.part_number
and b.stock_date > '2013-12-01'
and a.part_number like '9%'
Which would work on almost any database with the same structure (date format may vary), would render to something like this in Access:
select inventory_part_master.part_number, warehouse_stock_locations.qty
from inventory.part_master, warehouse.stock_locations
where ((inventory_part_master.part_number = warehouse.stock_locations.part_number)
and ((warehouse.stock_locations.stock_date > #12/1/2013#)
and (inventory_part_master.part_number like "9*")))
Only without the whitespace... try running that on any database but Access. It will say, "wha?!?" And that's a trivial example. Try it with a big query.