Haven’t found this setting in Marshmallow.
Do you use the Android messenger app?
When enering the number / name, there’s a SIM-card icon on the right side. If you tap on that, you should have the option to chose the other card.
Besides; if nothing else helps and you don’t want to delete the contact, as you would like the keep the SMS, you could rename it as well (e.g. add a “2” to the name) and than enter it anew.
PS (Edit): As I use the “Silence” app, that does not seem to have a dual-sim support, I just found out now, when giving the new Android messenger app a try.
You mean when I enter the message? When entering a name that sign doesnt appear. But Ill try the renaming trick… probably the last that could help. If not the programme lacks that funcion of simple redoing this simple assignation.
I think this really depends on the app you use for SMS. In Signal, long-pressing the send button lets you switch the SIM card. This also works within a conversation / thread!
So if it doesn’t work in your messaging app, choosing a different app might help.