If two were normal, I would think that it's adequate.

The reason they repeat the tests, is mostly because for some people it takes time to learn how to do them and than they get abnormal results.

In MG there can be worsening in the second test as compared to the first, so they may sometimes do it a third time just to be sure that there is such a trend.
Or sometimes they will not do more than one, if the patient is on a verge of a crisis and they don't want this effort to push them over the edge.

I would tend to think that a normal MIP and MEP measurement during significant respiratory difficulties would very much go against respiratory muscle weakness as the cause.