Did you prune Niobe to the ground this spring? Usually Niobe is considered a type 2, which for me starts blooming on last year's wood in early June, and will continue for about 6 weeks. Jackmanii is a type 3, cut to the ground each spring and started its bloom cycle last week.

I think you could treat Niobe as a type 3, but you'd miss the huge flush she gives in the early spring.

So they do roughly bloom at the same time?

Niobe was just new this spring so I have not pruned her.
The tag said to prune low the first spring, a little higher the second and then prune at about 3 feet thereafter. Seems a weird bit of instruction to me but close enough to "hard" pruning?

Are you recommending that I do NOT plant these together?

I have another spot for the jackmanii, but it gets more shade and I have to paint the wall there yet.

Niobe has always been treated as a type III in my garden. It flowers later than it would otherwise but still has an impressive flush of blooms when it does bloom. When either will bloom for you will depend on your growing conditions, nonwithstanding your growing zone, since growing zones just give you the average lows and highs for your area. They don't take into account humidity, sunlight, temperature swings, and a multitude of other factors that can affect when clematis will bloom.

I treated "Niobe" as a pruning group 3 the first year in my garden and then a 2 the second year and it was magnificent in it's 3rd year for me this year!!..