Good point...I used the ESX and non-ESX licenses... a pair as the source and single-node as the target...all separate keys this way. I will have to test it out and see if any issue if using the same serial, but if you go single node on the target they did provide the extra license.
First time I deployed my 2 nodes, I setup a random serial number, like I used to do before, but I could not set the licenses… No error message either, but key and serial didn't match so it was ignored.
Now, the installation documentations mentions only two different serial numbers : The original one which does not look like it should be changed (4079432-74-1), and the one for the second node which is 4034389-06-2.
And this is the same documentation for ESX or non-ESX platforms.
I used to do the same...using different serials for each node but like you said the license adds but won't work if it doesn't match. The license file on the simulator site has these 3 serials and keys for each.