Brief Answer:
Can be used together, but long term use of lorazepam is not advised.

Detailed Answer:
I read your question carefully and I understand your concern about drug interactions.

The two do not have direct interactions with each other. The one concern about using both is that they both individually increase sedation, so using them both may make you drowsy. Other than that they can be used together. In fact when treating anxiety disorders they are at times started together to continue later only with amitriptyline (amitriptyline needs some weeks to work so lorazepam will do the job in the meantime). So the easy answer to your question is yes, as long as not making you feel too drowsy they can be used together.

That being said though, long term use of lorazepam in general (with or without amitriptyline) is not advised. With time they create addiction, develop tolerance (same dose is not as effective, need to increase), have side effects like memory loss and confusion. So sooner or later you should discuss with the doctor treating you for ulcers about finding an alternative, it is not something which is normally used for nausea and vomiting, it is an off label use. The longer you continue it the harder the withdrawal process will be.

I remain at your disposal for other questions.

---