It applies to both. In the designer, there is a drop down box for you to choose the type of printing method you want.
DTG is not available on every item though. And yes, you can place two designs that could take up most of the shirt if you wanted.
11.8 X 11.8 is the max design size you can submit. However, depending upon the garment, you can either enlarge or reduce it a certain percentage. You can do this directly on the designer as well as when you are creating garments for your spreadshirt site. Your customer, however, can not change the size of those garments (unless you have a designer on your site and enable that design). Hope that makes sense.