Twin Spell doesn't say anything regarding the summon spells but clearly states that a spells effects happen twice and summon spells have "Effect: One or more summoned creatures...". So you'd essentially cast the summon spell twice, both with the same result - either 1 or more summoned creatures and so you'd get either 2 creatures (1+1) or many creatures (more than 1 + more than 1), with no two of them more than 30 feet apart.
I think the rest of the text is there just to handle the other types of spells that can be twinned, so you should be good.