How are you planning to direct the freezer's condensor heat to the vent? Although I surely haven't seen them all, I don't recall running across a typical freezer, either chest or upright, with a fan-forced condensor. The typical method for chest units is that the condensor is embedded in the cabinet exterior wall so the waste heat can counteract sweating. Upright units have a passive "bedspring" condensor on the rear (with an anti-sweat loop around the door) ... or beneath for self-defrosting models with the condensate pan sitting atop it to promote evaporation.

Doesn't seem doable to me. If both run at the same time, whichever airflow is weaker may backflow to the other. If you intend that only one would run at a time, how would that be controlled?

How do you plan on capturing the heat? Our chest type freezers simply use the outside of the case as a heat sink. There are no fins or anything like that. The outside simply gets a little warm when it's running.

I think the uprights may be the same or have an external coil like an older refrigerator.

I don't think any of them (outside the commercial space) use any sort of fan as a condenser cooling system. So you'd probably negate any energy savings by rigging up fans to blow the air outside. Not to mention when you exhaust to the outside the air gets replaced from air from the outside. You may end up with a net energy loss.

First there was a suggestion to put this dumb box in MY garage/shop. With the laundry idea, I remember the heat from the freezer in our last house. I don't know what freezer we're buying yet, so nothing intelligent to say about the heat source. I thought I'd figure that out when I got the freezer. I'm good at making things. But I appreciate those comments about capturing and moving the heat.
As to "may backflow the other way", I doubt it. The outlet at the Y will have the least resistance to airflow going only three feet. Secondly if at any time the airflow reversed (pressure going from a positive to a negative)on either input to the Y, the flap will close.
As to energy used, I'm saving energy in lots of other ways in the house. The energy used in this scenario isn't the issue. If I have to put this thing in MY garage/shop I'm holding you guys responsible!
Are there any code issues to be aware of?

Our freezer (it's a chest type) doesn't really put out much heat at all. The cabinet is warm and that's about it. Not hot to the touch at all. Only a commercial type with the compressor and such on top would make a difference. And those may be able to be relocated somewhere else.

Ours was down the basement (now outside due to a flood in the basement - it was floating on the water) and I don't remember it making any difference in the temperature of the room. Neither in the winter or the summer.

Not sure which type you are thinking of but I wouldn't worry about it.

You're going to waste more energy in your attempt at accomplishing this than any saving you may be imagining. From your description, you're not even capturing the heat. You're just dumping it out of the dryer vent...which will require a fan, which require energy to run, etc. Saving on AC, then....is that the idea? I really don't understand why you want to do this. Unless there's more to it than that, I think it's silly.

I think what you'll find is that the freezer will generate enough heat in the enclosed space to make it run longer and I think (IMHO) that you'll have a hard time re-directing/capturing the heat to send it elsewhere. Just my HO..

Maybe I imagined more heat coming out of a freezer than is real. In my last house/garage the freezer was actually next to a side by side fridge. I remember the garage being a little warmer walking by there. Could be that the fridge was responsible for most of the heat. If there is a heat issue I now realize that it's easy enough to deal with after the remodel. Thanks for everyone's input.