Short answer: Read the PhD candidature rules for those two institutions; at best, they might allow you to go part-time at each institution. A better option would be to apply for suspension of candidature in NZ.
Longer answer: Most PhD programs have a set of rules for students that set out rules/expectations for the amount of hours the student will commit to the program each week. In cases where this is specified, usually the stipulated hours will be commensurate with a job ---i.e., a full-time candidature would be approx 36-40 hours per week and a part-time candidature less. Usually there are specific rules for taking leave from the program, and you get about the same amount of leave as in a job. In some cases, approval will be required for outside study or work that impacts on the student's ability to meet the required time commitment. Even if specific approval is not required, it would usually be considered to be a breach of the rules if a student does not put in the stipulated hours when they are not on approved leave.
If such rules are present in one or both of these programs then it is difficult to see how you could do both programs full-time. You might be able to apply for part-time candidature at one or both of the institutions in order to cut down the hours, but it would probably be difficult to get approval for this.
In view of your circumstances, let me offer an alternative. Since you are prevented from attending the university in New Zealand, your best bet here would be to apply for a suspension of your candidature for one year, so that you don't have to pay tuition for that year. Suspension of candidature means that you do not lose your place in the program, but you take a temporary break with the expectation that you will return to candidature afterwards. In present circumstances, where the pandemic is preventing you entering the country, such an application would probably be viewed favourably, and I suspect it would be granted. A one-year suspension of candidature would allow you to attend the other university for a year and see which you prefer. If you decide you prefer the program in Europe you can then apply for withdrawal from your university in New Zealand.