ok, so if you assign an interface an IP of 192.168.1.5 /30, I would assume that .4 is your network, .5 and .6 are your hosts and .7 is your broadcast. Then a new network of 192.168.1.8/29 where .8 is the network address, .9 is a host, which happens to be the interface. .10-.14 are also hosts, which would give you a total of 6 hosts and .15 would be the broadcast.
That seems like it should work... if the router knows it is allowed to use the zero subnet, which would be 192.168.1.0/30, which would have .0 as the network, .1 and .2 as hosts and .3 as the broadcast. Otherwise, it may think there is an overlap and not allow your addressing.
was ip subnet-zero also configured on your device?
Yes, both ip subnet-zero and ip classless are enabled.
I could have made life simpler I guess by going for 192.168.10.1 /29 for Eth0 and 192.168.1.1 /30 for Ser0.
But I wanted to test and see it for myself to see if routers can be configured with a 192.168.1. network with only the mask separating/distinguishing the last octet.
I even tried it with 192.168.1.1 /29 for E0 and 192.168.1.1 /30 for S0, but no luck. Maybe there is something wrong with mindtechcom's routers or IOS. The router says that the E0 address overlaps with the S0 address.
You might be confused about the purpose of the mask, because you thought that it would be possible to give the same IP adress to multiple interfaces but with different masks.
Actually it's not possible, because the mask in itself is not an identifier for the interface, it's the IP address that identifies uniquely each interface. The mask main purpose is to identify which part on the IP address is the network part, and which is the host part. This lets the interface know which subnet it belongs to.
Regarding your initial question, I agree with Jared about the subnet ranges, but I don't see why the "ip sunet-zero" would have an impact. I don't think so, because your subnets are:
192.168.1.4 /30 (which 192.168.1.5 /30 belongs to).
192.168.1.8 /29 (which 192.168.1.9 /29 belongs to).
I don't know what's wrong with your config,
I see that you have marked your question as answered. Hopefully it was just a fat finger in your config or the fact that you tried to have the same IP on two interfaces.
I just set up your original explained config with 192.168.1.5/30 on one interface and 192.168.1.9/29 on another interface in my test lab and had no issues.
What did you find to resolve the problem?