a single vac rear booster might not be the best route for you parallel twin vac machine ,
you could ...but if you combine the internal vacs exhausts ... and connect it to a rear single vac you have 200cfm going through a 1.5"
opening motor that itself inly produces about half the cfm ... it might be bottle neck of sorts .
I think the bast route would be to make it a triple vac parallel machine , there is probably room inside the pro plus to fit an additional vac ... but to do it with an external single vac booster u need to get a port added to the waste tank in some location ...
i think i would go through the lid . So you would just have a 1.5"( or 2" ) tank connector in the lid ... to connect the booster . And either have a screw cap or a plug to fit over it when not using the booster .
You could also fit the tank connector in the lid but use the machines hose connector to connect the booster ... and connect you vac hose to the lid ... the advantage of this might be if you fit a two inch connector ... where as the original pro plus just had a 1.5 i think . If you do this you would need to add a water deflector so as incoming water clears the upfacing stack pipes of the alltec ...
like in the video below .
but if you u just connect to the original outlet u dont want the deflector as it would suck up water .
And the booster ... get the airflex one .... cause its 100 less that the ashbys
https://www.youtube.com/watch?v=FPVb0vugdpk