Hi Nellyhill, They won't twine together naturally, but will criss cross each other somewhat naturally, though not properly - you'd have to assist where you want branches to go. Different varieties have different branching traits and different growth speeds, but that's not to say your two might be similar enough. It's more likely they will look like two plants side by side. No they won't choke each other out, though as roots are twice as many, make sure you feed the two root systems ie a little extra food.

Colours don't matter. They will be happy together. What may suit you is a multi-grafted plant with between 2 and 4 varieties grafted onto the one rootstock. Send an email to Barry Schlueter - barry@barryschlueter.com. Barry will be most helpful and give you lots of local information and should be able to supply a multi grafted plant if you want one. See link as well.
Good luck and let us see how it all works out with some pictures.
Cheers, Brian.

Here is a link that might be useful: American Hibiscus Society