Hello,
To answer your question briefly, unfortunately, your intended application will not work. RUTX09 has a single mobile modem, meaning that two SIM cards will not operate simultaneously and that the switching will still result in disconnect, as first, data disconnect has to be detected, next connection to a different ISP has to be established.
In your case, an external antenna, different frequency bands or radio access modes could be experimented with to try and achieve better connection quality.
Best regards,