I'm not sure precisely what you were reading-but that is definitely not how things go with our group.
Admittedly, currently we all live together.
However, I do go out for overnights with one or the other partner alone. In those cases I ALWAYS have a goodnight and good morning conversation with whichever partner isn't with me (AND the kids).
Also, it's common for us to take a moment (like when one of us is in the restroom) to send a text message to partners who aren't with us at various times during the day.
If it's more than ONE night away-not only is there always a goodnight and goodmorning conversation, in addition to periodic "we are separated for a few minutes" texts-but I will make a point of having a midday conversation with whoever isn't with me as well.
If it's more than one night-I leave a card or note for the partner left behind-and it's common for them to slip one into my bags before I go.
If it's out of state trip-I make a point of mailing a postcard and sending several decently sized emails regarding the trip.
This is common decency in regards to our relationships. We usually see one another EVERY DAY because we live together-so more than a day apart is unusual and deserves extra special tlc.
There are other people I've read on here that do not live with their partners and don't see or talk to each other every day and therefore it's less critical to them to keep in touch when with another partner.