Load balancing using EIGRP

[maracambus]

I have 2 serial connections between 2 routers. They work like a champ on different subnets. (10.1.1.0/24 and 10.1.6.0/24)
When I try to put second serial interface of a router onto the same subnet (like on page 436) I am getting error:

Main(config-if)#ip add 10.1.1.4 255.255.255.0
% 10.1.1.0 overlaps with Serial0/0

What am I doing wrong?

Here is config of interfaces:

interface Serial0/0
bandwidth 1000000
ip address 10.1.1.1 255.255.255.0
delay 100
clock rate 4000000
!
interface Serial0/1
description Link to R4 (second)
bandwidth 1000000
ip address 10.1.6.1 255.255.255.0
delay 100
clock rate 4000000

In addition, router EIGRP does not see both links to other router. But the other one sees them fine.

From Main:
10.0.0.0/24 is subnetted, 9 subnets
C 10.1.1.0 is directly connected, Serial0/0
C 10.1.2.0 is directly connected, Serial1/1
D 10.1.3.0 [90/30720] via 10.1.1.2, 01:09:24, Serial0/0
D 10.1.4.0 [90/30720] via 10.1.1.2, 01:09:24, Serial0/0
D 10.1.5.0 [90/20514560] via 10.1.2.2, 01:10:05, Serial1/1
C 10.1.6.0 is directly connected, Serial0/1
C 10.1.7.0 is directly connected, FastEthernet0/0
D 10.1.8.0 [90/20517120] via 10.1.2.2, 01:03:12, Serial1/1
D 10.1.10.0 [90/33280] via 10.1.1.2, 01:08:50, Serial0/0
D 172.168.0.0/16 [90/35840] via 10.1.1.2, 01:07:16, Serial0/0

From Neighbor:

10.0.0.0/24 is subnetted, 9 subnets
C 10.1.1.0 is directly connected, Serial0/0
D 10.1.2.0 [90/21024000] via 10.1.6.1, 00:14:30, Serial0/1
[90/21024000] via 10.1.1.1, 00:14:26, Serial0/0
C 10.1.3.0 is directly connected, FastEthernet1/0
C 10.1.4.0 is directly connected, FastEthernet0/0
D 10.1.5.0 [90/21026560] via 10.1.6.1, 00:14:30, Serial0/1
[90/21026560] via 10.1.1.1, 00:14:26, Serial0/0
C 10.1.6.0 is directly connected, Serial0/1
D 10.1.7.0 [90/20514560] via 10.1.6.1, 00:14:30, Serial0/1
[90/20514560] via 10.1.1.1, 00:14:26, Serial0/0
D 10.1.8.0 [90/21029120] via 10.1.6.1, 00:08:14, Serial0/1
[90/21029120] via 10.1.1.1, 00:08:14, Serial0/0
D 10.1.10.0 [90/30720] via 10.1.4.2, 00:13:53, FastEthernet0/0
D 172.168.0.0/16 [90/33280] via 10.1.4.2, 00:12:18, FastEthernet0/

What's wrong?