great you’re using Onyx!
In the model figure you sent, there is just one variable (the eta) which are measured by two indicators, however, if I read your text correctly, you have two independent measures which don’t combine to a common factor score, is that correct? Or do you want to measure “attitudes” as a common factor?
If not, I would first set up the model as it is given in your representation with both variables separately, and then use the other variable (at pretest) to predict the change by a regression path. For example, you could set up the model for explicit attitudes, and then add the implicit attitudes at pretest as a predictor variable and connect it to the eta_2, the latent change, to see how strong implicit attitudes predict the change in explicit attitudes. For the other direction, you could then set up the model with implicit and explicit attitudes exchanged. This is just a very quick suggestion from what I understood, I hope it helps a little.