I'm using a host-only network with VirtualBox on macOS Sierra, my guests are Ubuntu. My Host (192.168.56.1) can ping both Guest A (192.168.56.100) and Guest B (192.168.56.101). Each guest can ping Host, but guests can't ping each other.
Guest A routing table:
$ route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 192.168.56.1 0.0.0.0 UG 0 0 0 enp0s3
192.168.56.0 0.0.0.0 255.255.255.0 U 0 0 0 enp0s3
Guest B routing table:
$ route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 192.168.56.1 0.0.0.0 UG 100 0 0 enp0s3
169.254.0.0 0.0.0.0 255.255.0.0 U 1000 0 0 enp0s3
192.168.56.0 0.0.0.0 255.255.255.0 U 100 0 0 enp0s3
Because Host can talk to guests and each guest can talk to Host, I get a feeling that the host is not forwarding traffic between the guests.