Chris, as long as you are opening something, I would probably opt to open the combo (your Peavey Tour 115?), and disconnect that speaker, and then run the 215 as is, not to affect the tuning of the box, as it is designed for the TWO 15's ...
in this case, seeing as you are using EITHER the combo speaker, OR the 215 cab, I would probably opt to install an exterior jack for the internal combo speaker as well, so I wouldnt need to open the box to make the connect/disconnect each time, and then run everything through the external speaker jack on the amplifier... that way I can still use the combo alone in smaller situations, where the single 15 is sufficient ... this would give you some modularity also, and there would never be a chance of mistakenly using a second cab and lowering the impedance beyond what the head is capable of ... and should you decide to find a second 8 ohm speaker (and it has two jacks) you could just piggy-back your combo speaker off of it, again for your 4 ohm load ...
... just an expansion on earlier suggestions, but something to consider in your situation ... JMHO