Two trials setting a precedent (4,4)
I believe the answer is:
test case
'a precedent' is the definition.
(I've seen this before)
'two trials' is the wordplay.
'two trials' can mean two replacements for 'trial'.
'trial' becomes 'test' (synonyms).
'trial' becomes 'case' (as in a legal case).
'test'+'case'='TEST-CASE'
'setting' is the link.
(Other definitions for test case that I've seen before include "Lawsuit whose decision will form precedent" , "Court precedent" , "Legal action where the outcome sets a precedent" , "Legal action setting a precedent" , "Legal precedent-setter" .)