Let's say we have this long and easy sentence:
iPhone 6 Plusは速くなって、(かつ)大きくなった。
iPhone 6 Plus became faster, and (at the same time) became larger.
You can omit the first なる like this:
iPhone 6 Plusは速くて、(かつ)大きくなった。
iPhone 6 Plus became faster and (at the same time) larger.
Note that, in this form, the comma is optional. 「iPhone 6 Plusは速くて大きくなった」 is also fine, and this is the answer to your question.
The reminder is to avoid confusion. We can even omit this て and say like this:
iPhone 6 Plusは速く、大きくなった。
iPhone 6 Plus became faster and larger.
This simple and bold sentence is typically suitable for an ad. But in this form, comma is important. Without the comma after 速く, this sentence suddenly appears as ambiguous and confusing, because 速く may serve as an adverb which modifies 大きく:
[?] iPhone 6 Plusは速く大きくなった。
iPhone 6 Plus became quickly larger. [?]
iPhone 6 became larger (and it happened sooner than expected). [?]
This sentence would eventually be understood correctly with the aid of the context, because the topic is iPhone. But when we just hear "はやくおおきくなる", it is usually interpreted as "to grow quickly".