[css-logical] Split sentence in two.

Author: fantasai

css-logical-1/Overview.bs

@@ -341,8 +341,8 @@ Flow-Relative Box Model Properties</h2>
   are paired using the element’s own
   [=computed value|computed=] [=writing mode=].
   Although the [=specified value=] of each property remains distinct,
-  paired properties share a [=computed value=],
-  which is determined by [=cascading=]
+  paired properties share a [=computed value=].
+  This shared value is determined by [=cascading=]
   the declarations of both properties together as one;
   in other words, the [=computed value=] of both properties in the pair
   is derived from the [=specified=] value of the property declared with