No, Funguy, I suggest you think again, lol. If the pot is at least 2 feet deep and 12 inches across, then you can plant one in it. Ville de Lyon gets 2.4 m. high, but Bill Mackenzie is a tangutica type, and wants to get 22 feet high, with a rootball to match that sort of growth. Passiflora also requires good root room.
If you want 2 clematis in one pot, as long as its the size mentioned above, you could use smaller ones, such as The President and others with an eventual height of 8 feet or so.

Had to have a chuckle! :)) Bamboo is quite right! I wouldn't want to plant Bill Mac. in my garden let alone in a pot...lovely though he is! He is rampant! However, there are some lovely Clems for pots if you search the Clematis Nurseries, or ask pm me and I'll give you a list. There are also some lovely, really beautiful new Passifloras, some of which are a bit smaller than Cerulaea and the other well kent vars. I don't have any, but I recall seeing an article...think it was in RHS magazine 'The Garden' a few months back, so try on their website.

I went to measure and the pot measures about 30 inches across and is about 2-3 ft deep.

Still, from what you have said i take it the plants would not be happy as they could be if grown in their own separate large pots.

I have some small beds in my garden but due to 2 pine trees and a birch the beds are quite shaded and the soil type is sandy and dry partly due to the tree's plus I am nr bracknell,berkshire and i think the soil profile we have is podzol. Hence i am container growing mostly.

Thanks for both your helpful advise.

Oh no, not podzol! I just looked it up! had no idea what that meant but it looks poor! Makes me feel grateful for my waterlogged cold clay!

Anyway, yes, lovely big pot suitable for two clems if they are the right size, but not Bill Mack. Clems all have big root systems and that one is huge. I dont' know about the root balls of passifloras though...sorry. Lovely big pot though and you could grow some bedding to trail over the sides in there too! Sounds like a big enough pot to have a sizeable Acer in it...I think that's what I would put in a lovely big pot like that, an Acer Palmatum and a clematis florida to grow through it...but that's me not you! :))

Sounds lovely Karen! Sorry, im just showing off because we learned about soil profiles at college a couple of months ago...just flexing the old brain muscle :))

I have a small acer palmatum in another pot, its in a 6 litre pot and the acer palmatum is about 18 inches high quiet dwarf in structure...i could repot that into the big pot? But i'm dubious - Maybe this should be another question...but, would it be bad practice to pot up from say a 6 litre pot to a 40ish litre pot?

Well, usually we repot up one size as you already know. I suppose if you repot into that much bigger a pot you could get windrock problems perhaps, but to be honest I'm a bit of a 'give it a go' gardener! Then again, don't you think the little tree will look lost in the pot?

...and acers are quite slow growers, so it's not like it would fill that pot in a couple of years. But if that's what you want to do.....personally I'd keep the little tree in it's small pot until it's too big and then move it to a bigger pot, but not that big!