They share office space, own stakes in the other's companies. It sounds like an incubator, no? These solutions are appearing all over the place. Are incubators now a good idea? (YC is NOT included in that category)
The major difference this time around is that the incubators are going for much smaller dollar amounts: they're doing YC magnitude investments. I'm not sure if it'll end up being a profitable experiment or not, but I understand why people are trying.