I want the wall to be the thickness of two courses, but rather than
actually building two separate courses, could I just use the blocks
on their sides?

It would be much easier and less labour intensive just to buy blocks of
sufficient width. Although 440x215x100 is the common block size, you should
be able to find 440x215x215 in stock at a builder's merchant and they will
order you some if they don't have any.

Don't forget to read up about how to design walls and their foundations. The
wall needs to be designed, not just thrown up. You also need to decide on
whether you want aerated or dense concrete. Personally, I would just use
aerated, especially as it will be rendered, so appearance is less important.
The only advantage of dense blocks is really just resistance to surface
damage and better appearance. In terms of structural strength, they're just
fine. They are also slightly less likely to kill you if the wall falls down.

Many of these blocks, such as the Celcon Standard 440x215x215 I would use
for this application, can not only be used externally, but are also suitable
for use in foundations, too.

Christian.