I think I would agree with Dr Mayhem that the easiest solution is a second identical pedal. But, if you have a $300 rack effects unit or similar high-dollar pedal, this may not be feasible.
More clarification may be needed; What I hear is that you have one source (a guitar) that is split with a Y-connector or ABY into two different pedal chains, but those different chains must share one or more pedals.
The closest thing I can think of without soldering your own custom effects-loop switcher is two "true-bypass" effects loop selector pedals and two Y-patch cables. Plug the "single" end of each Y into the in and out of the pedal or pedal chain you want to share, and then one of the "split" ends of each Y into one effects loop, and the other split from each into the other selector. Now, by hitting one or the other selector, you can punch the pedal into one or the other loop. Because the selector pedals will be true-bypass, the use of the Y-connectors shouldn't cause a problem; the output side of the effects loop as well as the input will be disconnected when you punch the pedal out of a particular chain, so there shouldn't be any "bleed-back" of signal.
The problem will be that you won't want to have the pedal punched into both loops at once, because then your signals will be recombined through the pedal and you'll get some octaved signal in the guitar amp and some normal signal through the bass amp. If you need to switch quickly, you'll need to be able to hit both switches with the same foot at the same time; not impossible, but sometimes tricky.
Understand that store-bought true-bypass effects loop selectors start at about $85 for this Keeley model, and you'll need two of them. So, if the pedal you're trying to loop costs less than $170, it'd be cheaper to just buy a second one.