Adobe recommends avoiding using both legacy implementation and DTM on your web page as it can sometimes cause unexpected behavior.
Common issues when implementing simultaneously are:
If you run into such errors, you can troubleshoot by checking if your library is managed by Adobe.
If it is managed by Adobe, choose a different option. Usually, depending on how you want to manage your library, you can either use the ‘page code already present’ option or the ‘custom’ hosting option.
Also, verify that you use different tracker variables for the implementations. Using the same tracker variables affects your ability to reference s objects in your implementation. This can cause discrepancies in data collection.