400W HPS light and Portable A/C Exhaust ducting help!

Have a 900 BTU Portable A/C unit inside the grow tent (36''x22''x72'') to keep things cool from the 400W HPS light. The 440 CFM 6'' inline fan just arrived and wanted to configure it to replace the Portable A/C unit IN the the tent. Basically, I want the portable A/C out of the tent (in my room next to the tent) and the inline fan INSIDE the tent.

Here's where the problem is, there is a window kit that allows for ONE exhaust duct to be connect to it so I wanted to use a "Y" connector to connect both the A/C unit AND the fan's exhaust ducting together, and have that flow out the window. My question is, if the inline fan is blowing much more air and has a stronger flow than the A/C unit, will it cause the exhaust of the A/C unit to move in the reverse direction (back into the A/C unit)? I've attached a picture to make it more understandable.

if your worried about back flow into the ac, put a backdraft dampner on the ac side of the Y.

i also suggest hard piping as much as possible for the best air flow.

I'm thinking I can just have the fan blow the hot air from the tent out to the room, then the A/C can take care of the rest and blow it out of the window. Or is that too inefficient?