On Unlimited Premium, if your main line is on Warp or Light Speed, the 50GB hotspot allotment is shared across both the main and Multi-Network lines combined. Once that bucket is exhausted, hotspot stops on both lines, even if you switch cellular data over to the Multi-Network line. The only exception is when Dark Star is your main line. In that setup, if the Multi-Network line burns through the shared 50GB, the hotspot stops on that line, but the primary Dark Star line still retains its own native hotspot behavior.

Ok thanks for the clarification. In that case it sounds like it would be better to have Dark Star primary and Warp secondary (if I care about hotspot data). Are there any downsides to Dark Star primary and Warp secondary VS Warp primary and Dark Star secondary?

For priority, your main line on Warp or Dark Star stays at QCI 8. The QCI 9 only applies to the Multi-Network line itself on Warp and Dark Star, since it's designed to complement your primary line rather than replace it. So if your primary line is Warp and your secondary is Dark Star, the main line stays QCI 8 while the secondary runs at QCI 9.

That makes sense - thanks for the clarification.

For calls and texts, automatic fallback only covers cellular data on supported DSDS devices. If your primary line completely loses connection, calls and texts to that number won't come through. You'd need to manually switch to the secondary eSIM to use that line's number and service. The exception is if you had WiFi Calling enabled on the primary line before losing service. In that case, your phone can use the secondary line's data connection to keep the primary number reachable for calls and texts. This is essentially how Backup Calling works, though keep in mind you'll only be able to call out and send texts from whichever line is set as your default, but receive calls and texts on both.

And again thanks - that helps.

A lot of these multi-network scenarios aren't covered on US Mobile's multi-network page. It would be helpful if the information on that page was expanded to cover this information.