I think what you are trying to do is called a LAG on the SG300. This option is located under the "Port Management" tab and you can manage which ports you want to be in the LAG under "Link Aggregation" and there you will see "LAG Management" where you can edit your LAG 1 with the 2 ports that needs to act as one 2 Gigabit port.
I hope that was able to help you as that is what I have gathered from your question. Please reply back if you still need more assistance.
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