increase MAX_ASSETS_FOR_BUY_EXECUTION (#1733)

Partially addresses #1638

Still need a better solution to allow devs to have better control of
this.

---------

Co-authored-by: Keith Yeung <kungfukeith11@gmail.com>
Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>

polkadot/xcm/xcm-builder/src/barriers.rs

@@ -52,7 +52,7 @@ impl ShouldExecute for TakeWeightCredit {
 	}
 }
 
-const MAX_ASSETS_FOR_BUY_EXECUTION: usize = 1;
+const MAX_ASSETS_FOR_BUY_EXECUTION: usize = 2;
 
 /// Allows execution from `origin` if it is contained in `T` (i.e. `T::Contains(origin)`) taking
 /// payments into account.