I guess the answer to this depends on what you mean by the LAN ports on the router.

If you're talking a SOHO device, those 4 LAN ports are actually a switch. If you connect two of them into an existing hub or switch you'll set up a nice loop and kill your network (SOHO stuff doesn't come with STP to guard against mis-wiring).

If you mean separate interfaces on a proper router, you could probably get away with hooking them into the same hub or switch if you want to -- the router won't forward Ethernet frames between its interfaces, so there's no loop. I can't think of any situation where you would want to actually do this though. The only one which springs to mind (having two subnets in use on the same hub/switch and wanting to route between them) can be handled by just giving the router multiple IP addresses on one interface which is connected to the hub/switch.