You need an amplifier with speaker output from the power amp.
The Harley Benton is a 60 watt cabinet. Your Katana & GX won't be outputting more than about a watt, if that.
The Katana looks like it ought to be able to do it, but it doesn't have a speaker out, only line level. It has power amp in, but not out.
INPUT jack: 1/4-inch phone type
POWER AMP IN jack: 1/4-inch phone type
REC OUT/PHONES jack: Stereo 1/4-inch phone type
EXP PEDAL/CTL CH1/CH2 jack: 1/4-inch TRS phone type
AUX IN jack: Stereo miniature phone type
USB port: USB B type
AC IN jack
One problem with using the existing Katana to output to an independent power amp is that it will already include speaker emulation, meaning you'll get two lots of 'speaker sound', one emulated, the other real.
You could get a tech to have a look at the Katana & see if it would stand a buffered new output to be able to run both cabs at once. I wouldn't recommend DIY, you need to match impedances properly, or you risk blowing up the amp.