Here is a hypothetical:

If i were to get an HT-1RH and use my special 6 ultra combo as a cab, would there be a way for me to a/b between the blackstar and VHT? like use the vht for cleans and the blackstar for drive?

Also, what would be the best way to run pedals through both? Just a/b them between both amps so that when i switch amps, i a/b the line of pedals as well?
Unless your combo has stereo inputs, no. Most amps just have speaker outputs, no inputs at all.

You could A/B them IF your amp has a way for you to disconnect the internal speaker, AND if you had a pedal like a Radial Headbone VT. But for the price of one of those, you could just get a good stereo cab used, or even a mono cab for your Blackstar and just use your VHT normall.
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How would you even play an HT-1 through the Ultra 6? Does the Ultra 6 have a speaker input inside the cabinet? Because I don’t see one on the outside.
The smart way to do it would be to buy a second speaker cab. Then you can just A/B the amp inputs like a normal person.
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you would have to modify your combo since, from what I can see, the speaker has no external input and is directly wired to the amp section.

there is a a/b cab switch pedal by palmer (around the same price as the radial headbone) but I don't think it's worth the money depending on your setup. a normal switch pedal, a second cab and a second mic might be around the same price but more flexible.