The Motordyne exhaust come with it's own Y pipe, photo below. You can pair it with the ART test pipes. Y pipe you've purchased don't think it can be paired with the exhaust. I'll ask of there is anything to be done, but I doubt. Probably the easiest way would be to sell it to someone else
I emailed them quite a few years ago about this, apparently they can put 2 HFCs further back somewhere, I think where those 2 small straight pipes are after the Y pipe if I remember correctly. Custom order though as it's not on their website.
Oh!! Thats interesting!! Maybe its worth me enquiring!!