Is it safe to use one antenna for transmitting (main antenna PL connector) and another for reception connected to ANT IN connector at the backside panel?
According to the user manual published here, page 11, the receiver antenna IN and antenna OUT connectors are located before the TX/RX antenna switching relay.
Something like this:
Transmitter -------------------- Relay ---- PL connector
Rcv - RX-IN - Jumper - RX-OUT -----
So you can connect a preamplifier between RX-OUT and RX-IN, or get the RX-OUT antenna signal to an external receiver rather than the IC751.
So, if you get another antenna into RX-IN, you will get a very strong signal directly into your receiver every time you transmit (due to both antennas being very near). I wouldn't recommend this.
It is safe IF you take precautions.
If too much transmitted RF gets into the Icom's RX antenna input:
You will transmit spurious signals. The 751A (like most transceivers) shares stages in the radio with receiving and transmitting. When the signal exceeds the hold-off threshold on switching diodes, the diodes leak transmitter RF picked up by the receiving antenna into the transmitter's early stages. This results in RF feedback causing the radio to oscillate on spurious frequencies. Your signal might also sound distorted.
YOU WILL DAMAGE PARTS. The most likely damage is to the attenuator pad and early filter stages including diodes. It takes about 1/2 watt or more to cause damage, but considerably less than that to create the first condition.
There are even commercial products that you can buy to prevent this. One is the KD9SV Front End Saver.