Do you mean that you need to make sure one message goes through the flow before the next is allowed in? If so this example flow uses a Delay node in a mode where it can be used to queue messages and release them when it is told the previous one has completed. In this flow the node named Process Taking 5 Seconds simulates some process that needs to be completed before the next one can be allowed in. If you feed your Split node into this it will handle the messages sequentially.
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