IAP's will automatically join the same cluster if they are part of the same L2 broadcast domain. You could have two IAP clusters at the same site providing they are in separate L2 domains however I expect this is not recommended.
The IAP105 and IAP305 are not able to be part of the same cluster as the AP305 requires a minimum of Aruba InstantOS 4.3.1.0 which is not supported by the IAP105.
You can disable auto-join on each of the clusters and manually add the MAC of each IAP you wish to join if they are in the same L2 domain.
This being said however from a RF point of view, you have a split RF domain so the contention domain will be increased if the AP's are within earshot of each other.
What is the requirement for the additional network? Can this not be something which is implemented on the existing hardware?