What's the current input from each leg of a portable generator into my main panel?
No. You can't do that.
You have to have an interlock in the main panel.
Your idea is clever, and you think &quot;Why not a checklist&quot;?  The answer is people got hurt doing it that way, often enough that they changed the Electrical Code to stop the carnage.
In a crisis people do not &quot;rise to the occasion&quot;. They descend to the level of their training, and if they aren't well trained, the result is chaos.  Code requires your system be built to endure through that.  Simple as that.
Since your inlet equipment must be at the main house anyway (you can trench a separate cable to send it to the garage if you really want to), you can simply select commercially available inlets that include amp meters.  You are interested in amp meters; there's no need to monitor voltage (plus, it's pretty easy to do with other devices such as a Kill-a-Watt).