input :- test1 : "CRIS3-AB; CRIS3-AB; ; ; SOECS; SOECS; ; ", test2 : "; ; Related FOX; Related FOX; ; ; Trace; Trace", Desire output : " CRIS3-AB;CRIS3-AB;Related FOX; Related FOX; SOECS; SOECS; Trace; Trace"

I am not sure how your description matches the desired output, but I got some code to share:

a!localVariables(
  local!test1 : "CRIS3-AB; CRIS3-AB; ; ; SOECS; SOECS; ; ",
  local!test2 : "; ; Related FOX; Related FOX; ; ; Trace; Trace",

  local!test1Splits: split(local!test1, ";"),
  local!test2Splits: split(local!test2, ";"),
  if(
    a!isNullOrEmpty(local!test2Splits[1]),
    a!update(
      local!test2Splits,
      1,
      local!test1Splits[1]
    ),
    local!test2
  )
)

I am not sure how your description matches the desired output, but I got some code to share:

a!localVariables(
  local!test1 : "CRIS3-AB; CRIS3-AB; ; ; SOECS; SOECS; ; ",
  local!test2 : "; ; Related FOX; Related FOX; ; ; Trace; Trace",

  local!test1Splits: split(local!test1, ";"),
  local!test2Splits: split(local!test2, ";"),
  if(
    a!isNullOrEmpty(local!test2Splits[1]),
    a!update(
      local!test2Splits,
      1,
      local!test1Splits[1]
    ),
    local!test2
  )
)

Shon this error :

Interface Definition: Expression evaluation error [evaluation ID = I48HU] : An array of components may only contain components. Received Text at index 1. 

Again, I am not sure what you are looking for and you initial ask lacks any level of details and context. My recent podcast episode might be of interest for you: https://appian.rocks/2023/12/20/episode-13-how-to-get-your-questions-answered/

The code snippet is an expression rule, and will not work in an interface.

I want, If no value available before semicolon symbol of local!test2 then a value from local!test1 should take place.

OK. Feel free to adapt my code to fit your needs.