This spell's associated magic effect is left without any keyword linking it to the Twin Souls perk, so no two Staadas could be summoned simultaneously. As a side effect, however, conjuring Staada while having any other summons (or thralls) dismisses the summons (or thralls), even if the Twin Souls should allow you to keep one of them. However, you can conjure Staada first and afterward conjure another summon. With this order you can have two summons at the same time, one of which will be Staada.