I think "The two of them will watch the match today." would be a better translation for this sentence.
Yes! One couldn't get away with that English out loud. It would sound like "They too will ..."
"They two" is not proper grammar. I agree with oruga_fantasma and GeraldMath4 for their reasons given.
Lāua was probably used to familiarize us to a dual pronoun.
Remember, Hawaiian has singular, dual and 3+.
"They two" is bad English. It should be "These two", "those two" or as suggested by orgua_fantasma, "the two of them". But not "They two". That just sounds terrible, and as GeraldMath4 rightly pointed out, ambiguous when said out loud.
This feels really clumsy to translate, I'm sorry.
I get there are differences in the languages but when this is translated like it is here, it becomes confusing.