Can you use this if you're using Tret?

No, not if you’re using them in the same areas. Tretinoin is a more potent retinoid than retinaldehyde, so there’s no point in using this product on skin where you’re already applying tretinoin. However, you can use retinaldehyde or a less potent form of retinoid on skin that doesn’t tolerate tretinoin well. For example, some people apply tretinoin to their face, but use retinaldehyde on their neck/décolleté.