No, you do not.
There is an input seal on the transfer case and an output seal on the transmission - both seal against the transfer case input shaft. The junction between the two cases, however, should not be sealed - in fact, you may find that there is a "notch" in one case or the other at the bottom. This is there to allow any leaks to actually make it out from in between the cases so you can tell that there is a leak that needs to be serviced.
If you seal that junction, you will not be able to determine if/when you have a leaking transmission output seal or transfer case input seal.