I often plant two group 2s or group 3s together on trellis or obelsiks to get a contrast of colour and flower shape and it works well. Putting two different pruning groups together would be a pruning nightmare.
For plants to cover just one panel would have to be something like Pagoda - http://www.clematis.hull.ac.uk/new-clemdetail.cfm?dbkey=529 and Romantika - http://www.clematis.hull.ac.uk/new-clemdetail.cfm?dbkey=396 but if those don't float your boat, you could use the same website to do further searches for clems that only get too between 2 and 2.5 metres and are both in pruning group 2 or 3.
Whatever you choose, you need to keep on top of training in the new growth to get a maximum horizontal spread across your trellis.
Montana will get to 7 to 10 metres so needs more scope to grow.
Have fun whichever you choose.