Can the main card see the short message sent by the secondary card?

I can't. The main card will not receive the SMS from the secondary card, but the SMS from the secondary card will be sent to the secondary card. The primary card and the secondary card do not conflict. Individuals only need to deduct the package fee of the secondary card from the main card. They just use a common deduction account, and everything else is independent, so the SMS sent by the secondary card or the main card of the phone bill is invisible.