>>> chad (north, close to the libyan border) - sudan
If you mean Mourdi Depression then this is quite a radical route for a pair of Synchros which will have to be very heavy. You're more likely to meet smugglers and rebels up here than any tourists. A mate did it recently in the other direction (direct to Bilma) but he always travels way off the map anyway. There's not even a line on the Mich map and that's saying something as many of the Mich black lines barely exist.
I've heard one or two people do the south route you decribe to get to East Africa. Otherwise all you can do is the obvious Nig, Alg, Tun, Lib, etc.