Can two packets propagate on the same communication link? Can two packets, say from host A, propagate on the same communication link while the first one is still on its way to the next point, say host B?

On the same physical link, no. The source needs to finish sending a packet before it can start a new one. Packets are atomic and cannot overlap.

Physically, with the limited propagation speed of a signal, the destination may still be receiving the previous packet while the source has already started transmitting the next one. With a 10 Gbit/s link and a propagation speed of 200,000 km/s, a single bit is just 2 cm "long". For a minimum sized Ethernet frame that's just 13.5 m, so a longer cable could actually hold one or more entire frames. [ _figures corrected_ ]

On a logical link, consisting of multiple physical links, it is normal for the source to send many packets while the destination is still receiving previous packets. In the TCP transport-layer protocol, the _window size_ defines the amount of data that can be "in flight".