ok that makes sense, just was confused because you were getting errors for each platform.
I will say this,
Twitch wont allow you to stream higher than 6000 (unless you have the Enhanced Broadcasting for multitrack video turned on, in which you dont have control of your bitrate for that anyway cause twitch controls it based on variety of factors)
So assuming you have the Enhanced turned off, you can put 7500, but twitch will only take 6000, maybe 6500 but def not 7500
Now youtube on the other hand can handle ALOT higher than 7500, its all based on what you set up your stream key for in YT Live Stream dashboard. if its saying not enough for youtube, my guess is your default stream key in the dashboard is set up for a higher resolution and your not pushing enough bitrate to stream at that resolution. I dont know if the plugin is capable of changing those setting for you. Might wanna open a ticket for it.
Another thing to look at is your internet speed. The way i understood how the multistream plug in works in the stream they had about it, it uses entirely local resources, meaning your PCs processor, RAM, GPU and internet upload speed. So for instance your outputting to Twitch and YT, so take your bitrate 7300 and double it, thats the total bitrate your trying to output. If your upload speed cant hit it, then you need faster internet on lower your bitrate ALOT.
If the upload speed isnt the issue, then its your pc not being able to handle the workload of processing multiple streams locally. In that case if your wanting to multistream then their plugin wont work for you until the release the version that allows you to control bitrate for each output, similar to the Multi RTMP plugin that everyone uses that doesnt have SE multi or Streamlabs Ultra. Another option is a cloud multi service like restream, where they take the workload off ur pc and its done in the cloud. If your only doing 2 platforms and dont wanna worry about the performance stuff, that might be the best route for you.