If you are using this plugin and you are sending 9,000 Kbps to the 2 platforms, you will need to be able to send 18,000 Kbps.
Now the 'get from OBS' option just prevents the encorder from doing double encoding. You are using the settings of OBS so it is less processing but does not change the fact you are still sending to 2 platforms therefore 2 x 9000 Kbps
With Restream, Castr, Splitstrem and similar providers you will send one stream, 9,000 Kbps, to them and they will "restream" the stream to the 2 or more platforms for you.
Now the general recommendation is that is you are pushing a stream of X Kbps you should try and have 2 to 2.5 that amount in upload capacity. The reason is you want to ensure you have headroom for other activity that might happening on the network and or fluctuation in the upload speeds.