I see, I was speaking from a previous Printful experience with Etsy. (which I have found to be totally different from Shopify)
I have yet to tried Printful and Printify on Shopify, but I did a bit of digging and realize the process is totally different from Etsy (obviously). Judging from what I read it's should be possible to have both together, as Printful and Printify would each publish their own standalone product, thus there shouldn't be any conflict.
In other words, if you have a design that needs to be on products of both Printful and Printify, you would have to add the design separately on Printful and Printify - and from there push these products to your Shopify store.
Hope this helps.