Thanks  daggett: I wrapped script2 as a method and passed script1 as parameter in script2

pipline.groovy

node('Node1'){
 def loadedScript1 = load('./Script1.groovy')
 loadedScript1()
 //..some code
 def loadedScript2 = load('./Script2.groovy')
 loadedScript2.script2(loadedScript1)
}

Script1.groovy

def script1(){
 println 'Script1 works'
 //..some code
}
return this;


Script2.groovy

def script2(Script script1){
 println 'Script2 works'
 //..some code
 script1()
}
return this;