Running two lines on one phone is straightforward once you know which two settings actually matter. Get them right before you fly and the whole arrangement becomes invisible - your phone just works, on the right line, without you thinking about it again.
Most of the confusion around dual SIM comes from people assuming it's more complicated than it is. It isn't. It's two settings, checked once, and then genuinely forgotten about for the rest of the trip.
The two settings that matter
Every phone with dual SIM support lets you assign a default line for data separately from a default line for calls and messages. That's the entire trick. Set your travel eSIM as the default for mobile data, and leave your home SIM as the default for calls and texts. Everything else - which line shows first in your contacts app, which colour dot appears in your status bar - is cosmetic.
It's worth saying plainly because most confusion about dual SIM comes from treating it as one setting rather than two. People find the toggle for calls, set it once, and assume data follows automatically. It doesn't - the two are independent, and both need setting.
Setting it up before you fly
On iPhone, this lives in Settings > Mobile Data, where you'll see "Default Voice Line" and a separate data line selector. On Android, it's usually under Network & internet > SIMs, with separate toggles for calls, texts and mobile data. In both cases, do this the same week you install your eSIM - not at the gate.