A blend knob only applies to signals in parallel.
Here are some possibilities:
1. You like how they sound as separate pedals, and you want to put them into one box. You're able to dial in the sound you like with the existing controls on both pedals.
2. You like them as separate pedals, but you wish you could get less octave than you're currently getting.
(In each case you have the choice of controlling both circuits with a single bypass switch, or having separate bypass switches for each circuit.)
Option one is dead simple. Just build both circuits into one enclosure.
The second option is less simple because it's a design project instead of just a build. There are multiple ways you could go about it, and you'll have to make some decisions about what you prefer. For example, it might be as simple as modding the Tentacle circuit to give you the amount of octave you want. Or you might prefer to add a clean blend to the Tentacle. But the clean blend
might sound wrong, because the Tentacle adds gain and tone shaping, not just octave, in which case you might want to blend between the Tentacle and a booster (or a dirt circuit, or an EQ, etc.). This is all totally do-able, but I recommend you try it on the breadboard first because parallel blending can be unpredictable sometimes. Especially with octaves.
So is it option 1 or option 2, or is it something else?
Edit: Sorry. I missed this sentence somehow:
Would it be as simple as just using a pot to bypass the octave circuit?
Yes. It could be that simple. Try it and see. It should be easy enough to wire up an experiment with your existing pedals, and see if you like it.