RTFM. (Obviously, you may need to download it first.)
Quick precis. The onboard Eminence loudspeaker is rated at 8 Ohms. The external loudspeaker jack is expecting a load of 8 Ohms. If you run both at the same time, the jacks are in parallel. Hence, the combined parallel loading will be 8/(1+1) = 4 Ohms.
SUGGESTION
Is there any way that you can get to test your amplifier through loudspeaker cabinets loaded with the G12-65, Cannabis Rex and so on?
CAUTION
To do this safely, you need to disconnect the onboard speaker, connect to the external test loudspeaker enclosure, then power up as normal. Similarly, before testing another loudspeaker, you need to power down fully before disconnecting the 'speaker cable.
*
EDIT: Judging by this thread and your other one about Mullard valves, it appears that you are trying to "Marshall-ise" your Fender HRD. A quicker and less expensive way to achieve this already exists - the Marshall Guv'nor pedal. (Run it into the clean channel of the Fender.)
Officiandos usually prefer the big, black mk1 version. The later, Chinese-made Guv'nor Plus version adds a "Deep" EQ pot. This is handy for dialling in some simulated low end cabinet thump.