Hi Mieke.
Finding a ride partner for such a big tour is difficult. It can prove problems enough riding with a good mate, your with one another 24/7 and personell differences can get in the way.
I'm presently cycling to capetown and paired up with someone who wanted a partner for Western Sahara and Mauritania.
He liked to take an hour for breakfast, an hour for lunch, whereas I tend to eat more smaller snacks sort of on the go. He prefered hotels and auberges's whereas I prefered camping so it wasn't exactly a match-made-in-heaven. It's little things like this can lead to problems further down.
And to put the icing on the cake the main country he was concerned with (Mauritania) he got food poisoning through drinking unfiltered tap-water and ended up getting a lift through most the desert to Nouakchott.
Not sure if you've toured before but if not i'd strongly advice on doing some smaller ones first, more so to see what gear you prefer and what suits your needs. A 'trial' tour with the new ride partner would be good to test-the-water.
TIP: Most of the countries (on the west coast) were colonised by the french and it can prove so frustrating when asking for such simple things so if you can't speak the lingo, go on a french speaking course, this is something i wish i'd have done now!!..and to think I had a french girlfriend for two years!
If you can't get a ride partner then going solo is always an option, no reason for not going then.
email me if you'd like further advice.
All the best, Mark
wallisonwheels