Quick update: “Signal” has just added DualSIM support: http://sgnl.link/1LoIMUl
It defaults to SIM1.
SIM2 can (must) be selected by a long press of the send-arrow. This must be done every time you want to send a text with SIM2, but if you rarely use it like is the case for me, this works just fine. (I posted a feature request to have the app remember the selected SIM for each conversation - let’s see if they can add something like this some time.)
And it’s back to only one app for both secured messages and SMS texts 
UPDATE and correction: Signal already HAS a “sticky SIM” implemented:
- It does remember the last SIM-selection for a given conversation when sending a text.
- If the peer responds to the other SIM, this does not affect this setting.
- (And probably: testing the SIM behaviour by sending texts from one of my SIMs to the other of the two in my phone, did not seem to help identifying the actual real life behaviour of Signal …)