You can usually connect the modem output to more than one micro-controller RX input without problems.
You cannot normally connect more than one micro-controller TX pin to a modem input pin, unless you use an AND gate or some other extra circuitry. Even then, you will need to ensure that no two micro-controllers transmit at the same time.
.... unless maybe your micro-controller has an open-drain option on the TX pin, which few have.
Do not forget to connect a common ground connection to all devices.