They can't be participial phrases. There are no participles present. As I parse them, they are absolute phrases.
An absolute phrase is a supplemental noun phrase with a postpositive modifier. Strictly speaking, there is no noun in either of these phrases. Instead, each phrase has the word "some". Depending on your framework, you might label these as pronouns or as substantive adjectives. Regardless of label, they play the same role that a noun typically plays. The words "simultaneous" and "sequential" are also adjectives, here taking their ordinary attributive role even though they follow the words that they modify.
The two phrases are coordinated asyndetically, which means that there is no conjunction marking the coordination. Either "some simultaneous and some sequential" or "some simultaneous but some sequential" could serve the same purpose.