Routing – How to Route Two Routers on One Switch in Different Networks

You can, and you should. If the switches are stacking switches then you can even have the ports on 2 separate switches. It's a rather simple port-channel configuration on the router and switch. Cisco has a document explaining this, Configuring EtherChannel and 802.1Q Trunking Between Catalyst L2 Fixed Configuration Switches and a Router (InterVLAN Routing).

interface Port-channel1
 switchport trunk native vlan 10
 switchport mode trunk
 no ip address
 flowcontrol send off
!
interface FastEthernet0/1
 no ip address
!
interface FastEthernet0/2
 switchport trunk native vlan 10
 switchport mode trunk
 no ip address
 channel-group 1 mode on
!
interface FastEthernet0/3
 switchport trunk native vlan 10
 switchport mode trunk
 no ip address
 channel-group 1 mode on

And on the router:

interface Port-channel1
 no ip address
 hold-queue 150 in
!
interface Port-channel1.10
 encapsulation dot1Q 10 native
 ip address 10.10.10.1 255.255.255.0
!
interface Port-channel1.20
 encapsulation dot1Q 20
 ip address 10.10.11.1 255.255.255.0
!

!--- Output suppressed.

!
interface FastEthernet3/0
 no ip address
 channel-group 1
!
interface FastEthernet4/0
 no ip address
 channel-group 1

Sorry for answering rather old question.

Speaking about GNS-3: you can change the label of R1 or some other router if you stop the device (right-click the router, select stop), then right-click it and select configure.

Glad to hear you managed to add routes:

R4(config)#ip route 11.0.0.0 255.255.255.0 21.0.0.1

R5(config)# ip route 21.0.0.0 255.255.255.0 11.0.0.1

I think your problem is not solved in its entirety though - as long as R5 does not know how to reach network 10.0.0.0/24 (as a result R5 cannot ping 10.0.0.2 and vice-versa). The same goes for R4 and the network 10.0.0.0/24.

So, you should add a pair of routes:

R4(config)#ip route 10.0.0.0 255.255.255.0 21.0.0.1

R5(config)# ip route 10.0.0.0 255.255.255.0 11.0.0.1

After that all the IPs of your lab are pingable.

BTW, you could replace a pair of routes with one default route, the result would be the same. For R5 it would be:

R5#(config)ip route 0.0.0.0 0.0.0.0 11.0.0.1