Comment 6 for bug 1568750

This is not really true: "This worked previously in OTA 9 (I think also 9.1), but does not have an effect starting OTA10."

It works perfectly upon starting the Messaging app, only. Means workaround is very simple (I am not saying its comfy), just follow this procedure every single time you want to send SMS:
 - stop Messaging app (swipe up in the list of opened apps)
 - wait (with lot of messages it takes time to read the db)
 - start Messaging app
 - open the conversation
 - if selected SIM1 fits - do not touch it!
 - if selected SIM1 does not fit, select SIM2 - and DO not touch it anymore
 - write the message
 - send the message
Make sure you do the above in one "row" of actions, any interruption or screen lock or changing apps may mean a wrong SIM will be selected and you shall start all over.