Ah! I finally got it to work. I created 3 text layers: txt1, txt2, txt3.
For illustration purposes,
txt1 says: Hello there, its
txt2 says: morning
txt3 holds the expression to concatenate txt1 & txt2.
I but I also wanted txt3 to inherit the font style of txt1.
txt1=thisComp.layer("txt1").text.sourceText;
txt2=thisComp.layer("txt2").text.sourceText;
txt3=txt1+" "+txt2;
fontStyle=thisComp.layer("txt1").text.sourceText.style.font;
style.setFont(thisComp.layer("txt1").text.sourceText.style.font).setText(txt3);