Can I treat two different outcome measures as equal once standardised, even if they apply to different life-domains?

#1
I am about to launch an online study on the topics of self-control and goal-directed behaviour. To briefly provide an overview of the elements of my study that are relevant for this particular question (thus: this is not the complete design of the study):
  • Monday: pp constitute a personal goal, either in the domain of healthy eating or studying (they are free to choose themselves)
  • Tuesday/Wednesday/Thursday: pp are asked pursue their goal to the best of their ability
  • Friday: follow-up survey in which they report on their goal-directed behaviour over the past 3 days. For healthy eating we use the Adolescent Food Habits Checklist (AFHC, 23 items; forced choice format with a 3rd 'not applicable' option for some items, which means that unequal numbers of items may be used to calculate the mean score per participant), and for studying the Student Study Behaviour Inventory (SSBI, 20 items; 5-point Likert).
The issue is that we have to use different questionnaires to measure goal-directed behaviour for the two domains (we've tried to use the same outcome measure for both domains, didn't work). The idea is to calculate the z-scores of the AFHC and the SSBI, and use these standardised scores to create one variable: goal-directed behaviour. However, my supervisor and I are not completely sure if this is appropriate. The other option would be to run two studies, one for each domain, but we unfortunately do not have sufficient resources to double the sample size. Or, we could cut down to only one goal-domain, but this could result in people participating only for the money, but who do not truly identify themselves with their goal, which could cloud the results.

I am curious to hear your opinions on this matter!