by definition - an HTP merge wont work for a colour device!... an LTP merge would be more usefull on a colour device.
However, as previously stated - a switch function between 2 ports would be of more use as there are 2 controllers connected.

Merging usually requires hardware - ie: an Artnet box (node)
However, it is also possible with some software:
http://www.artisticlicence.com/index..._id=&order_id=

Art-Net II Protocol Release V1.4 Document Revision 1.4ak 9/11/09 (extract)
Data Merging:
The Art-Net protocol allows multiple nodes or servers to transmit ArtDmx data to the same universe.
A node can detect this situation by comparing the IP addresses of received ArtDmx packets. If ArtDmx packets addressed to the same Universe are received from different IP addresses, a potential conflict exists.

Merge is implemented in either LTP or HTP mode as specified by the ArtAddress packet.

When a node provides multiple DMX512 inputs, it is the responsibility of the Node to handle merging of data. This is because the Node will have only one IP address. If this were not handled at the Node, ArtDmx packets with identical IP addresses and identical universe numbers, but conflicting level data would be transmitted to the network.