Huldine and Will Goodwin are two different pruning classes of clematis so unless you plan on treating them both as type IIIs I would advise against planting the two together in any sized pot.

I would go for a pot at least 24 inches tall or larger and just as wide or so if you plan to grow Will Goodwin in a pot. The pot may restrict the size the plant will eventually get however.

A nice large flowered type 3 I have is Ascotensis.

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It may be a bit hard to find, and I planted it in the ground rather than a pot/tub. The clay pot contains a Sedum.
There were only 3 vines on the plant last year, and I trained 2 upwards and this one more horizontal & slightly downwards along the telephone wire that is tied between my brass headboard fence and a ground screw like you attach a dog's chain to. The fence leans inward due to the weight of the American Bittersweet vines and needs to be pulled toward the road.

The flowers are quite a bit larger than Huldine.

Marie