It means "should".

“这两位” equals "these two (persons)",so "这两位孪生兄弟" literally means "these two twins".Chinese idiomatic expression is different from English here.It stresses measure words.Usually in formal expression these measure words cannot be omitted.If you omit "这两位" here,then the sentence would sound weird.You can substitute "对" for "两位"(a pair of),which is also a measure word. “本当” is the abbreviation of “本来应当”,it can be simply translated as "should have","would have".

得到相同的机会,这对有着相同大脑的双胞胎会在大致相同的测试水平。 这两位 refer to the twins would have translated into 本当