I think there is no problem to use as specified single miter bend , only concern to check your required pressure drop for the system .if angular angle is more than 3 degree it will consider as miter bend as per ASME 31.3. miter bend may be single or multiple bend.
It is rare to find a 2-piece mitre in cabinet making. A stopped- or hidden-dovetail joint is more likely.
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An imperfect mitre is difficult to disguise. Due to angle mismatch or slide-by when clamping. A dovetail or mortise and tenon joint hide minor imperfections better.