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Yes, If the GSM modem gives out TTL logic serial output you surely can connect it directly to the Rx of micro controller.
But do make sure that it is TTL logic.
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.
Instead of external multiplexers, you can simply three-state the TX pins of the "other" microcontroller.
The problem not yet considered in your post: How do you decide which controller talks to the modem, how do you avoid collisions? But the same problem would arise with external multiplexers as well.
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