How long in between multiple mortgages?

I have been pre-approved at my credit union for a mortgage amount. I want to live in the house but rent the rest out, then rent it out fully when I move out to a new house. The credit union said they would give me a residential loan, and this situation would NOT constitute as mortgage fraud. I recently found a cheap house nearby I could live in, and another one I could get soon after to live in and rent out. The CU said that they issue 2nd, 3rd, and so on mortgages, but they did not state a period in between mortgages. I would like to get both houses with the same mortgage or one a FEW months after the other if need be. My question is do they give you a period of time on your first mortgage before you can get your second if they know that both will eventually be investment properties? (Ex: You must have 6 months of on-time payment before pre-approval of second mortgage)

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